Manuel Algarra

Juan Soto
email: soto@uma.es

Luis Pinto Da Silva

M Soledad Pino-González

J Enrique Rodríguez-Borges

Joelle Mascetti

Fabien Borget

Adel Reisi-Vanani

Rafael Luque
email: rafael.luque@uco.es

Insights into the Photodecomposition of Azidomethyl Methyl Sulfide: A S 2 /S 1 Conical Intersection on Nitrene Potential Energy Surfaces Leading to the Formation of S -Methyl-N -sulfenylmethanimine

Keywords: CASSCF, MS-CASPT2, Conical Intersection, Intersystem crossing, UV Photochemistry, Matrix Isolation

published or not. The documents may come

INTRODUCTION

Organic azides are energy rich molecules with several industrial and technological applications, known and used since the beginning of the last century. Azides are widely employed in a diversity of synthetic preparations, important components in click chemistry and as nitrogen source due to the facile loss of N 2 upon thermal or photochemical activation. [START_REF] Wentrup | Flash Vacuum Pyrolysis of Azides, Triazoles, and Tetrazoles[END_REF][START_REF] Moses | The growing applications of click chemistry[END_REF][START_REF] Wentrup | Carbenes and Nitrenes: Recent Developments in Fundamental Chemistry[END_REF][START_REF] Liu | Spectroscopic Characterization of Nicotinoyl and Isonicotinoyl Nitrenes and the Photointerconversion of 4-Pyridylnitrene with Diazacycloheptatetraene[END_REF][START_REF] Silva | Influence of Alkyne and Azide Substituents on the Choice of the Reaction Mechanism of the Cu+-Catalyzed Addition of Azides to Iodoalkynes[END_REF][START_REF] Meisner | Fluorescence of Hydroxyphenyl-Substituted "Click" Triazoles[END_REF] They also find applications in fields including the automobile industry (e.g. propellants in air-bag systems of automobiles) and semiconductor processing (as single source precursors of thin films), simplifying synthetic processes for the preparation of such materials. [START_REF] Liu | Perfluorophenyl Azides: New Applications in Surface Functionalization and Nanomaterial Synthesis[END_REF][START_REF] Lowe | Thiol-yne 'click'/coupling chemistry and recent applications in polymer and materials synthesis and modification[END_REF][START_REF] Hiki | Versatile and Selective Synthesis of "Click Chemistry" Compatible Heterobifunctional Poly(ethylene glycol)s Possessing Azide and Alkyne Functionalities[END_REF][START_REF] Yurtsever | Quantum Chemical View on the Growth Mechanisms of Odd-Sized Nitrogen Cluster Anions[END_REF][START_REF] Bera | Improvement of Photostability and NIR Activity of Cyanine Dye through Nanohybrid Formation: Key Information from Ultrafast Dynamical Studies[END_REF] Due to the significant interest of these molecules, numerous spectroscopic studies have targeted the fundamental understanding of decomposition products of organic azides and their intermediates supported by computational analysis. Both thermal and photochemical decomposition of organic azides have been extensively studied in solution and various matrices by photoelectron and FTIR matrix isolation spectroscopies and demonstrated to exhibit highly interesting and complex chemistries. [START_REF] Dyke | A study of the thermal decomposition of 2-azidoacetamide by ultraviolet photoelectron spectroscopy and matrix-isolation infrared spectroscopy: Identification of the imine intermediate H2NCOCHNH[END_REF][START_REF] Dyke | Contrasting behavior in azide pyrolyses: An investigation of the thermal decompositions of methyl azidoformate, ethyl azidoformate and 2-azido-N, N-dimethylacetamide by ultraviolet photoelectron spectroscopy and matrix isolation infrared spectroscopy[END_REF][START_REF] Pinto | Thermal Decomposition of Methyl 2-Azidopropionate Studied by UV Photoelectron Spectroscopy and Matrix Isolation IR Spectroscopy: Heterocyclic Intermediate vs Imine Formation[END_REF][START_REF] Pinto | Pyrolysis of 3-azidopropionitrile studied by UV photoelectron and matrix-isolation IR spectroscopies: Formation of ketenimine H2C=C=NH[END_REF] Imine derivatives have been observed together with reactive nitrene intermediates under laser flash photolysis. [START_REF] Wentrup | Reactivity and utility[END_REF][START_REF] Wentrup | Interconversions of Reactive Intermediates[END_REF][START_REF] Gritsan | Study of photochemical transformations of organic azides by matrix isolation techniques and quantum chemistry[END_REF][START_REF] Kubicki | Direct Observation of a Sulfonyl Azide Excited State and Its Decay Processes by Ultrafast Time-Resolved IR Spectroscopy[END_REF] Based on these decomposition patterns, various pathways were identified as depicted in Scheme 1. Type 1 pathways [START_REF] Dyke | A study of the thermal decomposition of 2-azidoacetamide by ultraviolet photoelectron spectroscopy and matrix-isolation infrared spectroscopy: Identification of the imine intermediate H2NCOCHNH[END_REF][START_REF] Dyke | Contrasting behavior in azide pyrolyses: An investigation of the thermal decompositions of methyl azidoformate, ethyl azidoformate and 2-azido-N, N-dimethylacetamide by ultraviolet photoelectron spectroscopy and matrix isolation infrared spectroscopy[END_REF] lead to imines as final products after N 2 loss via nitrene intermediate followed by isomerization. Alternatively, imines may be formed by a 1,2-H shift synchronous with N 2 elimination. Type 2 pathways [START_REF] Pinto | Thermal Decomposition of Methyl 2-Azidopropionate Studied by UV Photoelectron Spectroscopy and Matrix Isolation IR Spectroscopy: Heterocyclic Intermediate vs Imine Formation[END_REF] involve transfer of intra-groups (proton or alkyl) onto the electron-rich N and may involve a cyclic transition state. The thermolysis of azidobenzyl phenyl sulfide has been reported to generate a sulfenylimine product, in which a thiaziridine intermediate was proposed. [START_REF] Jarvis | Thermolysis of alpha-azido Sulfides, sulfoxides, and sulfones -Dependence of Mechanism on Oxidation-state of Sulfur[END_REF] This type of rearrangement represents the least common rearrangement among those described in literature and can be considered as Type 3 pathway in the present work (Scheme 1). The possible formation of the rearrangement product, according to Type 3 pathway, would be relevant because the obtained N-sulfenylimines (N-alkylidenesulfenamides) [START_REF] Bharatam | Electronic structure of N-sulfenylimines[END_REF][START_REF] Bharatam | Se-N interactions in selenohydroxylamine: a theoretical study[END_REF] are useful intermediates in the syntheses of antibiotics and other bioactive molecules. [START_REF] Esteban | From (E)-and (Z)-ketoximes to Nsulfenylimines, ketimines or ketones at will. Application to erythromycin derivatives[END_REF][START_REF] Lamy | Synthesis and reactivity of 4 ''-phenylsulfinimine-avermectin B-1 and 4 '-phenylsulfinimine-avermectin B-1 monosaccharide derivative[END_REF] N-sulfenylimines have a remarkable reactivity, with possible redox processes at the imine bond (i.e. reduction) and sulfur (i.e. oxidation) as well as nucleophilic additions to the same carbon and electrophilic additions at the nitrogen.

In this work, we aimed to investigate the photochemical decomposition of an organic aliphatic sulfide azide in order to understand the possibilities to couple such decomposition with the generation of interesting light-mediated compounds for organocatalysis. [START_REF] Nicewicz | Merging photoredox catalysis with organocatalysis: The direct asymmetric alkylation of aldehydes[END_REF] Sulfur-containing aliphatic azides are attractive since click chemistry applications may allow their use in nanotechnology related fields including functionalization and surface patterning as for other types of azides. [START_REF] Chen | Photoinitiated Alkyne-Azide Click and Radical Cross-Linking Reactions for the Patterning of PEG Hydrogels[END_REF][START_REF] Arumugam | Photoreactive Polymer Brushes for High-Density Patterned Surface Derivatization Using a Diels-Alder Photoclick Reaction[END_REF] Aromatic thio-azides have occasionally been described. [START_REF] Gritsan | Study of photochemical transformations of organic azides by matrix isolation techniques and quantum chemistry[END_REF][START_REF] Jarvis | Thermolysis of alpha-azido Sulfides, sulfoxides, and sulfones -Dependence of Mechanism on Oxidation-state of Sulfur[END_REF] but to the best of our knowledge, this is the first reported mechanistic study related to alkyl thio-azides that combine matrix isolation techniques and computational calculations.

R N ´R H N N R N ´R H R N ´R H N N R,R´= H, Me allyl 1 ,2 -H s h if t NH R ´R -N 2 Type 1 Type 2 N H N N H O O H CO 2 + CH 2 =NH + N 2 N H N N SPh Ph   N H SPh Ph N H SPh Ph Type 3 nitrene -N 2
Scheme 1. Different pathways in the thermic decomposition of aliphatic azides

RESULTS AND DISCUSSION

With the aim to elucidate the decomposition mechanism of azidomethyl methylsulfide (AMMS), the method of matrix isolation infrared spectroscopy was used in these studies together theoretical calculations. UV photodecomposition of AMMS. AMMS was irradiated under UV light, see Supporting Information (SI) details of the UV spectra of AMMS (Figure S1A) and the emission spectra of the used Hg lamp, (low pressure Hg lamp irradiating above 180 nm (Figure S1B). Isolation in Ar, N 2 and Xe matrices at 10K were recorded for the first time in order to estimate energy differences, barriers of inter-conversion and vibrational spectra of the different conformers of AMMS. Annealing of the matrices was also systematically carried out.

Pure AMMS was characterized by recording FTIR spectra in N 2 , Ar and Xe matrices at 10 K. Figure 1A displays the spectra of AMMS in Ar matrix (2040-2180 cm -1 ) showing an excellent quality spectrum with sharp absorption bands as compared to those under Xe or N 2 , where less matrix site effects are observed. In this spectral region are localized the bands corresponding to the asymmetric mode  a (N 3 ) of the pattern molecule, in fact, at least six intense bands are recorded at 2115, 2118, 2122, 2127, 2130 and 2134 cm -1 (Tables 1 andS1). The correlation between the calculated and observed bands is not straightforward. Furthermore, it is well known that some azides, e.g., methyl azide, give intense overtones and combination bands in this spectral region, which are more intense than the fundamental modes that originate them. [START_REF] Quinto-Hernandez | On the Interaction of Methyl Azide (CH3N3) Ices with Ionizing Radiation: Formation of Methanimine (CH2NH), Hydrogen Cyanide (HCN), and Hydrogen Isocyanide (HNC)[END_REF][START_REF] Khlifi | Absolute IR band intensities of CH 2 N 2 , CH 3 N 3 , and CH 3 NC in the 250-4300 cm -1 region and upper limits of abundance in Titan's stratosphere[END_REF] Thus, we cannot assign all the observed bands in the ~2100 cm -1 region to fundamental modes. Furthermore, for example, isomers 2 and 5 (Table 1 and Figure S2) have the same electronic energy and their fundamental frequencies are almost the same. Hence, we would expect to observe only five bands in this region of the infrared spectra. Therefore, we assume that the former six bands arise from five fundamental frequencies plus one combination band. Vibrational analysis in internal coordinates for isomer 3 (C s conformer) is given in Tables S2 to S5.

In the temperature range 10-30K, the energy barriers between all conformers are too high (between 1 and 3 kcal/mol according to our DFT calculations) to allow interconversion processes. Only few changes related to metastable matrix site vanishing could be observed. Table 1 lists the bands, which were observed in Ar matrix at 10 K and the calculated ones by MP2, of the most characteristic functional groups for the isomers of AMMS. The asymmetric stretching frequency of -NNN group, predicted for all conformers, lies in the region 2156-2166 cm -1 . The experimental spectrum reveals, as previously mentioned six bands, strongest bands which are extending from 2115-2134 cm -1 (Figure 1A). As it has been previously reported, [START_REF] Frankowski | Matrixisolation FTIR study of azidoacetone and azidoacetonitrile[END_REF] the bands show asymmetric shapes, additional splitting, and an appreciable change in the intensities from matrix to matrix. S2A and atom numbering for vibrational analysis in Figure S3. rocking modes (around 860 and 905 cm -1 ). Bands located in the 1200-1240 cm -1 range can be assigned to  rock (CH 2 ) and  twi (CH 2 ) modes. The recorded spectra were too noisy to observe weak bands below 800 cm -1 , which could correspond to deformations of NCH and NCS moieties. Spectra recorded in solid N 2 exhibited less bands as compared to that on Ar, with some missing weak bands. Table S1 summarizes all annealed experiments in N 2 matrices. These findings may indicate slightly lower barriers between conformers under N 2 as compared to Ar which could be related to low affinities of AMMS to form a welldefined local crystalline structure resulting in poorly defined sites, with a wide range of local geometries (matrix site effect, < 10 cm -1 , Figure S5).

After deposition, the matrix was subjected to a step-by-step annealing procedure (from 10 to 30K in Ar, Figure 2B), aiming to alter relative conformer populations to further support the interpretation of the experimental spectra. In Ar, the observed spectral changes were found to be minor and essentially ascribable to site conversion. Annealing experiments in N 2 were comparatively more informative (Figure S6), with six remaining bands at 2122(s), 2135(m), 2151(w), 2081(w), 2057(w) and 2095(ms) cm -1 , respectively, correlated to the  as (N 3 ) vibration of the azide group. Interestingly, a change in temperature generated intensity changes between bands at 2135 cm -1 (increasing when decreasing T)

and 2122 cm -1 (increasing with T).

The photochemical decomposition of AMMS was subsequently conducted under UVvisible light and studied by FTIR in Ar-matrix (10 K) at different times of irradiation (typically 0-120 min). After 10 min irradiation, IR vibrations of  as (N 3 ) and  s (N 3 ) at 2100-2200 cm -1 (Figure 2A) and 1180-1260 cm -1 (Figure 2B) started to disappear, while many new features due to photodecomposition products are growing. New peaks of increasing intensity were observed after 90-120 minutes of irradiation in the 2800-3300 cm -1 (Figure 2C) region (namely 2850, 2875, 2948, 3012, 3165, 3169, 3174, 3189, 3233, 3239 and 3293 cm -1 , (Figures S7 andS8). Clearly, these bands arise from of a new compound formed in the photolysis of the parent azide and could be assigned respectively to the stretching modes of the =CH 2 and -CH 3 moieties of such a new compound. In addition, new weak bands are observed in the range 682-748 cm -1 (Figure 2D). Infrared absorption bands of the H 2 C=N-moiety. The most featuring bands that are registered in the spectra depicted in Figure 2 correspond to two bands that appear in the 3100-3200 cm -1 region (Figure 2), whose intensities increase as time increases. In addition, it is observed that such absorption bands depend strongly on the supporting matrix and the material phase in which the spectra are registered, Figure S9 and Table 2, respectively. The infrared spectra of the parent molecule, methanimine (H 2 C=NH), show the same behavior for the asymmetric stretching mode of CH 2 . Table 2 collects the positions of the bands of methanimine calculated in this work and those ones given by one of us (FB) [START_REF] Theulé | Hydrogenation of solid hydrogen cyanide HCN and methanimine CH 2 NH at low temperature[END_REF] and other two independent groups. [START_REF] Jacox | The Infrared Spectrum of Methylenimine[END_REF][START_REF] Hamada | Pyrolysis of Amines: Infrared Spectrum of Methyleneimine[END_REF] The most striking data of this table refers to the C-H stretching mode of the H 2 C=N-moiety, which varies in both position and intensity as vary the experimental conditions. Therefore, we have assigned these two bands to the asymmetric stretching of CH 2 = (3238 cm -1 ) and  s (CH 2 )/' as (CH 3 ) (3233 cm -1 ) modes of Smethyl-N-sulfenylmethanimine, respectively, which are calculated at these wavenumbers (Table S3A-B). Theoretical study of the reaction mechanisms. Scheme 2 proposes a reaction mechanism for the formation of S-Methyl-N-sulfenylmethanimine after AMMS photodecomposition. Thus, according to the mechanism proposed in Scheme 2, to study the photodecomposition reaction, we have applied the CASSCF and MS-CASPT2 methods [START_REF] Roos | The Complete Active Space Self-Consistent Field Method and Its Applications in Electronic Structure Calculations[END_REF][START_REF] Roos | The multi-state CASPT2 method[END_REF] in conjunction with the ANO-RCC basis sets [START_REF] Lind | Main group atoms and dimers studied with a new relativistic ANO basis set[END_REF][START_REF] Lind | New relativistic ANO basis sets for transition metal atoms[END_REF] as implemented in the MOLCAS 8.2 program. 38 Table 3 collects the vertical transition energies of the electronic transitions calculated at the MS-CASPT2 level. The assignment of electronic transitions given in Table 3 is based on the state average CASSCF orbitals of the minimum energy geometry of AMMS given in Figure S10. According to the calculated excitation energies, upon exposition to UV-radiation, the S 1 state of AMMS is populated (Table 3), the next state is out of the range of the source applied. Given that the first step in the decomposition of azide derivatives [START_REF] Gritsan | Spectroscopy, and Computational Chemistry of Arylnitrenes[END_REF][START_REF] Soto | Conical intersections and intersystem crossings explain product formation in photochemical reactions of aryl azides[END_REF][START_REF] Aranda | An MS-CASPT2 study of the photodecomposition of 4-methoxyphenyl azide: role of internal conversion and intersystem crossing[END_REF][START_REF] Soto | Conservation of El-Sa ed's ules in t e P otol sis of P en l Azide: Two Independent Decomposition Doorways for Alternate Direc Formation of Triplet and Singlet Phenylnitrene[END_REF] usually gives the corresponding nitrene plus N 2 , we have built the linear interpolation [START_REF] Soto | Reaction Mechanisms of 2-Butanol Dehydration over a Phosphorus-Containing Activated Carbon Acid Catalyst[END_REF][START_REF] Soto | Photodissociation Mechanism of Methyl Nitrate. A study with the Multi-state Second-Order Multiconfigurational Perturbation Theory[END_REF][START_REF] Peláez | Dependence of Nnitrosodimethylamine Photodecomposition on the Irradiation wavelength: Excitation to the S-2 state as a Doorway to the Dimethylamine Radical Ground-State Chemistry[END_REF][START_REF] Ruano | Multiconfigurational Second-order Perturbation Study of the Photochem-ical Decomposition of Methyl Thionitrite[END_REF][START_REF] Soto | Application of Surface-Enhanced Resonance Raman Scattering (SERS) to the Study of Organic Functional Materials: Electronic Structure and Charge Transfer Properties of 9,10bis((E)-2-(pyridin-4-yl)vinyl) An-thracene[END_REF][START_REF] Peláez | A complete active space selfconsistent field study of the photochemistry of nitrosamine[END_REF][START_REF] Peláez | Photochemistry of protonated nitrosamine: Chemical inertia of NH(2)NOH(+) versus reactivity of NH(3)NO(+)[END_REF][START_REF] Algarra | Insights into the formation of N doped 3D-graphene quantum dots. Spectroscopic and computational approach[END_REF][START_REF] Arenas | Multiconfigurational second-order perturbation study of the decomposition of the radical anion of nitromethane[END_REF] that connects the S 0 minimum of the azide derivative (M 0 , Figure 3(a)1)) with its dissociation products (Figure 3b). The potential energy profile depicted in this figure indicates that there would be two stationary points on the S 1 (1 [START_REF] Wentrup | Flash Vacuum Pyrolysis of Azides, Triazoles, and Tetrazoles[END_REF] A") surface around the Franck-Condon point, that is, a minimum and a transition state. In fact, we have been able to optimize these stationary points, denoted as M 1 (Figure 3(a)2) and TS 1 (Figure 3(a)3), respectively. TS 1 is located at only ~14 kcal/mol with respect to M 1 and ~33 kcal/mol below the vertical excitation of the minimum ground state (M 0 ). In consequence, after surmounting a small energy barrier on the S 1 surface, AMMS dissociates into N 2 (S 0 ) and nitrene on the lowest singlet surface by decaying through a conical intersection (CI 1 , Figure 3(a)4). Therefore, the nitrene species would be generated in its lowest singlet state, which is an open shell structure (S 1 , 1 [START_REF] Wentrup | Flash Vacuum Pyrolysis of Azides, Triazoles, and Tetrazoles[END_REF] A", under C s symmetry) with two unpaired electrons localized in the p-type orbitals of the N atom. At the CASSCF level, we have found a minimum on the 1 [START_REF] Wentrup | Flash Vacuum Pyrolysis of Azides, Triazoles, and Tetrazoles[END_REF] A" surface of the nitrene species (M 2 , Figure 4(b)) which lies 32 kcal/mol above its triplet ground state (T 0 , 1 [START_REF] Wentrup | Carbenes and Nitrenes: Recent Developments in Fundamental Chemistry[END_REF] A", Figure 4(a)). In contrast, the vertical excitation energy of M 2 to its first excited singlet (S 2 , Despite S-methyl-N-sulfenylimine formation is not clearly observed in the experiments, the band registered at 3416 cm -1 (Figure 2C) could be assigned to the (NH) mode the imine. Therefore, we have searched for the reaction channel that would lead to this compound. The potential energy profiles of the interpolations are represented in Figure 6. The reaction path on the lowest energy surface is almost barrierless, the optimized Therefore, in principle, formation of the imine derivative would be energetically accessible. In consequence, if it is not clearly observed it must be concluded that formation of this species is hindered by some process. The only reason that we have found to explain why is not experimentally observed is the CI 2 conical intersection in the proximity of the nitrene that was formed in the first reaction step from the parent azide. This conical intersection acts as an attractor that guides the system to it and decouples the S 2 and S 1 states of nitrene leading, in turn, the system to the amine channel instead of the imine path. Concerning thermal dissociation of AMMS, starting at the ground state surface, the interpolation line suggests that there is a transition state (TS 0 ) on the S 0 surface and a T 1 /S 0 intersystem crossing (ISC 1 ) between the ground and lowest triplet states. In fact, these two points were optimized on the low-lying potential energy surfaces to demonstrate the reliability of our interpolation method. The structures and energetic of these critical points are given in Figure 7(a). Both TS 0 and ISC 1 yield nitrene and molecular nitrogen, the path through TS 0 leads to formation of (1 1 A') singlet nitrene and the channel through ISC 1 generates (1 [START_REF] Wentrup | Carbenes and Nitrenes: Recent Developments in Fundamental Chemistry[END_REF] A") triplet nitrene, in accordance with previous studies. [START_REF] Arenas | A multiconfigurational self-consistent field study of the thermal decomposition of methyl azide[END_REF][START_REF] Arenas | Potential-energy surfaces related to the thermal decomposition of ethyl azide: The role of intersystem crossings[END_REF][START_REF] Arenas | Nitrenes as intermediates in the thermal decomposition of aliphatic azides[END_REF] Unfortunately, triplet nitrene has not been observed in our experiments. The detection of triplet species would require, for example, EPR spectroscopy, that has been successfully applied to other nitrene derivatives. [START_REF] Wentrup | Trimethylsilylnitrene and Its Surprising Rearrangement to N-(Dimethylsilyl)methanimine via Silaziridine and Silaazomethine Ylide[END_REF][START_REF] Yang | Photodecomposition of Thienylsulfonyl Azides: Generation and Spectroscopic Characterization of Triplet Thienylsulfonyl Nitrenes and 3-Thienylnitrene[END_REF][START_REF] Korchagin | Steric Heavy Atom Effect on Magnetic Anisotropy of Triplet Tribromophenyl Nitrenes[END_REF] However, the calculations point out that formation of this species in the thermal decomposition of the azide is highly probable. Two factors reinforce the above assertion: [START_REF] Yarkony | Theoretical Studies of Spin-Forbidden Radiationless Decay in Polyatomic Systems: Insights from Recently Developed Compu-tational Methods[END_REF] (i) the magnitude of calculated spin orbit coupling constant for the crossing point that amounts to 40 cm -1 ; (ii) the geometrical distance between the calculated geometries of TS 0 and ISC 1 is only 0.05 Å. observed after irradiation, assigned to the -N=CH 2 moiety of this organic compound. The calculations on the reaction mechanism demonstrate that a S 2 /S 1 conical intersection in the seam of crossing of the S 2 and S 1 states of nitrene, which is localized in the proximity of S 1 minimum of nitrene, governs the reaction mechanism after N 2 elimination in the first step of the photochemical decomposition.

Transition E(eV) OS c Configuration d 1A' 2A' 6.02 0.0085 46% n  (NN) 1 *(NNN) 1 40% n  (NN) 1 *(NNN) 1 1A' 3A' 7.71 0.0040 82% p  (S) 1 *(NNN) 1 1A' 4A' 8.40 0.5209 25% n  (NN) 1 *(NNN) 1 44% n  (NN) 1 *(NNN) 1 1A' 1A" 4.57 0.0009 89% n  (NN) 1   *(NN) 1 1A' 2A" 6.47 0.0001 92% p  (S) 1   *(NN) 1 1A' 3A" 7.40 0.0054 90% n  (NN) 1   *(NN) 1 1A' 4A" 10.11 0.0017 61%(NNN) 1   *(NN) 1 18% n  (NN) 0 *(NNN) 1   *(NN) 1
To finish, rearrangement in the excited states (RIES), such as cyclization or hydrogen migration [see e. g. Refs. 60-68], are processes that are not considered in this work because the excitation energy at the Franck-Condon geometry, which populates the S 1 state, is well above the minimum (M 1 ) on the S 1 surface (Figure 7(a)). In consequence, after absorption of one photon, the system gains enough internal energy to surmount a small barrier which will lead to N 2 elimination, forming the nitrene species in its lowest singlet state (S 1 , 1 1 A"). In addition, once singlet nitrene is formed, it decays through a reaction channel that no needs extra energy to reach the final product, that is, S-methyl-Nsulfenylmethanime (Figure 7(b)). Franck-Condon

(a)

computed with a spin-orbit Fock-type Hamiltonian. [START_REF] Heß | A mean-field spin-orbit method applicable to correlated wavefunctions[END_REF][START_REF] Malmqvist | The restricted active space (RAS) state interaction approach with spin-orbit coupling[END_REF][START_REF] Ribbing | The optical absorption spectrum of the octahedral RhCl63-complex: Ab initio calculations of excitation energies and the effect of spin-orbit coupling[END_REF][START_REF] Arenas | Photodissociation mechanism of nitramide: A CAS-SCF and MS-CASPT2 study[END_REF][START_REF] Arenas | The ground and excited state potential energy surfaces of nitromethane related to its dissociation dynamics after excitation at 193 nm[END_REF] The active spaces that have been selected for each reaction have been carefully chosen to avoid erroneous answers. [START_REF] Soto | Multiconfigurational perturbation theory can predict a false ground state[END_REF][START_REF] Arenas | Carbene Formation in its Lower Singlet State from Photoexcited 3H-diazirine or Diazomethane. A Combined CASPT2 and Ab Initio Direct Dynamics Trajectory Study[END_REF][START_REF] Bernardi | An Ab Initio study of the Photochemical Decomposition of 3,3-dimethyldiazirine[END_REF] The analysis of molecular orbitals, geometries and vibrational frequencies have been performed

with the programs Gabedit, [START_REF] Allouche | Gabedit -A graphical user interface for computational chemistry softwares[END_REF] Molden 82 and MacMolplt. [START_REF] Bode | MacMolPlt: A graphical user interface for GAMESS[END_REF] The vibrational analysis in internal coordinates [START_REF] Pulay | Systematic ab Initio Gradient Calculation of Molecular Geometries, Force Constants, and Dipole Moment Derivatives[END_REF] have been performed with the GF theory of Wilson [START_REF] Wilson | Molecular Vibrations[END_REF] and the assignments of the fundamental modes have been done with the help of the vibrational potential energy distribution [START_REF] Keresztury | Alternative calculation of vibrational potential energy distribution[END_REF] with computational programs implemented by one of us (JS). [START_REF] Arenas | A method to improve the agreement between calculated and observed vibrational frequencies after scaling of a quantum mechanical force field[END_REF][START_REF] Arenas | Standard procedure to obtain a convenient set of scale factors in the SQMFF method[END_REF] DFT and MP2 calculations. The potential energy surface (PES) of AMMS has been investigated with the B3LYP [START_REF] Lee | Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron-Density[END_REF][START_REF] Becke | Density-Functional Exchange-Energy Approximation with Correct Asymptotic-Behavior[END_REF] hybrid functional and the 6-31G(d) basis sets. [START_REF] Ditchfield | Self-Consistent Molecular Orbital Methods. IX. Extended Gaussian-type basis for molecular-orbital studies of organic molecules[END_REF] Frequencies calculations were performed with the were optimized with the second order Møller-Plesset perturbation (MP2) method [START_REF] Møller | Note on an approximation treatment for many-electron systems[END_REF] and the def2-QZVPP basis sets [START_REF] Weigend | Accurate Coulomb-fitting basis sets for H to Rn[END_REF][START_REF] Weigend | Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy[END_REF] allowing the assignment of these stationary points as true minima and the determination of the corresponding vibrational spectra. All DFT and MP2 calculations were carried out with the GAUSSIAN 16 program. 95

a

  Labelling of isomers according to Figure S1. b Assignment according to vibrational potential energy distribution in internal coordinates of the AMMS3 isomer (Cs symmetry). c Definition of internal coordinates given in Table

Figure 1 .

 1 Figure 1. FTIR spectra in Ar, N 2 and Xe matrix (2000-2300 cm -1 spectral range) of (A) Trapped AMMS recorded at 10K. (B) Annealing of AMMS at 30K recorded at 10K

Figure 2 .

 2 Figure 2. FTIR spectra obtained after deposition of AMMS in Ar at 10K (black) and after irradiation under UV light for 30 min (blue) and 120 min (red), showing the decrease of all AMMS bands and the growing of new bands in the 3300-3100 and 760-680 cm -1 regions.

Scheme 2 .

 2 Scheme 2. Schematic representation of the proposed photochemical decomposition of AMMS.

1 1 A

 1 ', FigureS4(c)) amounts only to ~0.1 kcal/mol, which indicates that both singlet states of the nitrene derivative are strongly coupled. Interestingly, we have optimized a S 2 /S 1 conical intersection[START_REF] Galván | Analytical State-Average Complete-Active-Space Self-Consistent Field Nonadiabatic Coupling Vectors: Implementation with Density-Fitted Two Electron Integrals and Application to Conical Intersections[END_REF] (CI 2 , Figure4(d)), which is located at the proximity of the minimum of M 2 . It must be remarked that it is a conical intersection what efficiently couples/decouples two electronic states. Furthermore, the analysis of the branching space defined by the gradient difference and derivative coupling vectors given in Figure4(d)suggests that internal conversion through this conical intersection leads to formation of S-methylthiaziridine. Therefore, in order to confirm the mechanism proposed in Scheme 2, we have computed the profiles of the potential energy surfaces (Figure5), which leads from nitrene (CH 3 SCH 2 N, Figure4(b)) to N-sulfenylimine (CH 3 S-N=CH 2 , Figure 4(g)) passing by the aziridine intermediate (Figure 4(e)). It is observed in this figure that the path through the CI 2 conical intersection is barrierless. Once the S-methylthiaziridine intermediate is formed, the rearrangement to N-sulfenylimine needs to surmount an energy barrier. Due to the optimized transition state (TS 3 , Figure 4(f)) is 34 kcal/mol below the starting point (nitrene, M 2 ), the reaction can advance to N-sulfenylimine formation. To avoid any type of confusion to the reader, it should be note that the value of 34 kcal/mol mentioned above corresponds to the true optimized geometry, the point indicated in Figure 5 corresponds to the starting point (~14 kcal/mol below M 2 ) in the search for the true transition geometry. The state-average active space that has been used in these interpolations is given in Figure S14.

Figure 4 .

 4 Figure 4. CASSCF (14e,13o)/ANO-RCC optimized geometries of the species involved on the reactions after N 2 elimination of the parent azide: (a) T 0 triplet minimum (1 3 A"); (b) S 1 singlet minimum (1 1 A"); (c) S 2 singlet minimum (1 1 A'); (d) two views of the S 2 /S 1 conical intersection CI 2 : left: gradient difference vector; right: derivative coupling vector; (e) M 3 S-methylthiaziridine; (f) TS 3 transition state; (g) M 4 N-sulfenylimine; (h) TS 4 transition state; (i) M 5 imine.

  Thus, we can postulate that is a dynamic effect what favors yielding of the amine instead of the imine derivative.

Figure 5 .

 5 Figure 5. MS-CASPT2/ANO-RCC profile along the interpolation line connecting (1 1 A") nitrene with CH 3 SNCH 2 . C 1 state average (four states) CASSCF reference wavefunction. 14 electrons distributed in 13 orbitals. S i [S j ] on the left side of the graphic indicates the notation of the electronic state at the nitrene minimum M 2 (S i ) and at the final product M 4 (S j ).

1 Figure 6 .

 16 Figure 6. MS-CASPT2/ANO-RCC energy profiles along the interpolation line connecting 1 1 A" S 1 nitrene with CH 3 SCHNH. C 1 state average (four states) CASSCF reference wavefunction. 14 electrons distributed in 13 orbitals. S i [S j ] on the left side of the graphic indicates the notation of the electronic state at the nitrene minimum M 2 (S i ) or at the final product M 5 (S j ).

Figure 7 .

 7 Figure 7. MS-CAST2/ANO-RCC schematic representation of the reaction mechanisms leading to formation of (a) singlet nitrene and triplet nitrene (b) S-Methyl-Nsulfenylmethanimine. In square bracket, energy in kcal/mol.

Table 1 .

 1 Experimental (in Ar 10K) and calculated frequencies (MP2/def2-QZVPP) of the vibrational modes of AMMS isomers

	AMMS1 a	AMMS2 AMMS3 AMMS4 AMMS5 AMMS6	(Ar 10K)	Assignment b,c
	2311	2301	2320	2305	2301	2308	2115/2118/2122/	as(N3)
							2127/2130/ 2134 s
	1346	1342	1323	1341	1342	1355	1313	(CN)s(N3)
	1283	1272	1310	1284	1271	1267	1245/1220 s	rock(CH2)
	1245	1238	1191	1218	1238	1247		twi(CH2)
	1026	1025	1013	1004	1025	1025	1003	(CN)rock(CH3)
	980	987	992	990	987	987	959	wag(CH2)'rock(CH3)
	721	726	756	748	726	729	697/694/684	(CS)
	687	689	657	694	689	681	654/655/659

'(CS)(CNN)(NNN)

Table 2 . Infrared absorption bands (in cm -1 ) of methanimine (observed and calculated) Ar (10K) a Ar (4K) b Gas c Calc. d Assignment e

 2

	 1	3266	-	3264	3493 vw (0.93) f	(NH)
	 2	3162/3140	3035 m	3038	3203 m-s (0.95)	 as (CH 2 )
	 4 + 6		2966 w-m -
	 3	2987/2916/2886 2926 s	2915	3093 s (0.94)	 s (CH 2 )
	 4	1642	1641 s	1638	1678 m-s (0.98)	(CN) s (CH 2 )
	 5	1411	1453 s	1453	1492 m (0.97)	(CN) s (CH 2 )
	 6		1348 s-vs (1342) g	1371 vs (0.98)	(HNC) rock (CH 2 )
	 7	1186	1059 vs	(1055) h	1073 vs (0.98)	(HNC) rock (CH 2 )
	 8	1117	1123 vs	1131	1171 vs (0.97)	(CN)
	 9		1063 s	1058	1098 s (0.96)	(CH 2 )

a Ref. 31. b Ref. 32. c Ref. 33. d MP2/def2-QZVPP, this work. e Assignment according to the potential energy distribution obtained from the GF vibrational analysis and definition of internal coordinates given in Table

S5

. f In parenthesis ratio: [observed (gas phase)/calculated]. g Approximate value of the central minimum. h Average of the multiple peaks.

Table 3 . Vertical excitation energies in eV of the singlet states of AMMS (C s , MS- CASPT2) a,b

 3 MS-CASPT2 main electronic configurations of the excited states referred to the ground state configuration (see text). Only contributions with weight greater than 15% are included.

a C s optimized geometry. b Four state average CASSCF wavefunction, IPEA=0.25, Imaginary shift = 0.1. c Oscillator strength. d

ACKNOWLEDGEMENTS

MA thanks also This work was supported by FCT-Fundação para a Ciência e a Tecnologia through the CQM Base Fund -UIDB/00674/2020, and Programmatic Fund -UIDP/00674/2020, and by ARDITI-Agência Regional para o Desenvolvimento da Investigação Tecnologia e Inovação, through the project M1420-01-0145-FEDER-000005 -Centro de Química da Madeira -CQM+ (Madeira 14-20 Program). JS thanks Rafael Larrosa and Darío Guerrero for the technical support in running the calculations and the SCBI (Supercomputer and Bioinformatics) center of the University of Málaga for computer resources and projects CTQ2015-65816-R and UMA18-FEDERJA-049. LPS acknowledges funding from FCT under the Scientific Employment Stimulus (CEECIND/01425/2017). The publication has been prepared with support from RUDN University Program 5-100. Fellowship obtained from Gulbenkian Foundation is acknowledged.

transition state at the CASSCF level for this process (TS 4 , Figure 4(h)) lies only ~1 kcal/mol above of the singlet nitrene minimum (M 2 ). and filtrated. The solvent was removed under vacuum and the corresponding pure AMMS was obtained as a yellowish liquid (1.11 g, yield 95%) which absorbed at 298 nm (Figure S1). We confirmed the purity of these samples through NMR, the obtained spectra showed AMMS and traces of ethyl ether: FT-IR Matrix Isolation Spectroscopy. The experimental techniques used to obtain Ar and N 2 matrices have been described elsewhere. [START_REF] Borget | Photoreactivity on a water ice surface: cyanoacetylene (HC 3 N) reaction with atomic oxygen issued from the photodissociation of ozone (O(3)) at 255 nm[END_REF] The pre-mixed organic azide vapors with Ar and N 2 matrix gases were prepared at room temperature using standard manometric techniques with concentrations of 1:900 for all experiments*. Samples were continuously deposited onto a copper plate cooled down at 20 K. A FTIR spectrometer Nicolet series II Magna system 750 was used to record the spectra of samples cooled at 10 K by reflectance in the range 550-4000 cm -1 with a resolution of 0.125 cm -1 . The spectra were recorded immediately after deposition, after annealing of the samples, and after irradiation of the matrices with an OSRAM ® 200W high pressure Hg lamp equipped with a quartz envelope (all bands). Deposits were irradiated at 10K with a mercury lamp ( > 200 nm, average power 150 mW, total fluence: 0.72 J.m -2 ).

COMPUTATIONAL SECTION

CASSCF and MS-CASPT2 calculations. The contraction scheme applied to the ANO-

Supporting Information

Information related to the computational calculations are included here.

AUTHOR INFORMATION

manuel.gonzalez@staff.uma.pt (M. Algarra) soto@uma.es (J. Soto) rafael.luque@uco.es (R. Luque)

Table of Contents

Matrix isolation FTIR and MS-CASPT2 studies of the photo decomposition of azidomethyl methyl sulfide (AMMS) in Ar ( ) matrix at 10K: a S 2 /S 1 conical intersection on the nitrene potential energy surfaces explains the formation of S-methyl-N-sulfenylmethanimine.