- Email: [email protected]
Multiconfigurational second-order perturbation study of the photochemical decomposition of methyl thionitrite
Chemical Physics Letters 553 (2012) 17–20
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Multiconfigurational second-order perturbation study of the photochemical decomposition of methyl thionitrite Cristina Ruano, Juan C. Otero, Juan F. Arenas, Juan Soto ⇑ Department of Physical Chemistry, Faculty of Sciences, University of Málaga, E-29071 Málaga, Spain
a r t i c l e
i n f o
Article history: Received 13 June 2012 In final form 26 September 2012 Available online 12 October 2012
a b s t r a c t The photodissociation of methyl thionitrite (CH3SNO) has been computationally studied by means of CAS-SCF and MS-CASPT2 methods. The analysis of the multiconfigurational wavefunction corroborates the assignments of the absorption spectra, the visible and UV bands assigned to S0 ? S1(n, p⁄) and S0 ? S2(p, p⁄) transitions correspond to 1A0 ? 1A00 (nr, p⁄) and 1A0 ? 2A0 (np, p⁄) excitations, respectively. With respect to the photochemistry of CH3SNO, it is found that the potential energy surfaces associated with the low-lying excited states of this molecule (11A00 , 21A0 , and 21A00 ) are repulsive along the NO elimination coordinates. For this reason, population of such excited states leads to NO extrusion as the primary process, in agreement with experiments. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction S-nitrosothiols (RSNO’s) are biologically important because of their role in storing and transporting NO within the body and their potential medical use [1]. Recently, there is a growing interest in these compounds when they act as light-sensitive NO donor drugs, because they can deliver nitric oxide under the total control of irradiation at selected targets [2]. The photodissociation dynamics of the parent S-nitrosothiol molecule, methyl thionitrite (CH3SNO), has been previously studied [3–5]. The electronic absorption spectrum of methyl thionitrite shows a very weak band in the visible region around 500–600 nm and a stronger transition centered around 350 nm. The visible and UV bands are assigned to S0 ? S1(n, p⁄) and S0 ? S2(p, p⁄) transitions, respectively [3,4]. Irradiation of CH3SNO in the near-UV region yields CH3SSCH3 and NO, which points to an S–N bond cleavage [5] as the primary step according to the reaction: CH3SNO + hv ? CH3S + NO. Analysis of the NO photofragments after irradiation of CH3SNO at 450 [3] and 355 [4] nm has shown that the populated state at these wavelengths corresponds to a p ? p⁄ transition on a repulsive potential energy surface [3]. The multireference character of the wavefunction of S-nitrothiols was recently demonstrated by an ab initio study on HSNO [6] performed with the complete active space self-consistent field (CAS-SCF) method. We report now the multiconfigurational calculations of the electronic excitations and the dissociation potential energy curves for methyl thionitrite. It will be shown that S1, S2, and S3 states of CH3SNO correspond to repulsive surfaces with respect to NO elimination. Additionally, the assignment of the ⇑ Corresponding author. E-mail address: [email protected] (J. Soto). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.09.060
observed spectrum is supported by means of transition dipole moment calculations and analysis of the wave function. Furthermore, this Letter is of potential interest as a benchmark for calculations on S-nitrosothiols derivatives performed with other theoretical approximations. 2. Computational details Calculations have been performed with the complete active space self-consistent field (CAS-SCF) method and the multistate extension of the multiconfigurational second-order perturbation (MS-CASPT2) approximation [7,8] as they are implemented in the MOLCAS 7.6 program [9]. Geometry optimizations followed two strategies: (i) optimization at the CAS-SCF level of theory by computation of analytical energy gradients and (ii) optimizations with the MS-CASPT2 method by computation of numerical energy gradients. Two state average CAS-SCF reference wave functions were used in the MS-CASPT2 optimizations. Transition dipole moments were computed according to the CAS state interaction (CAS-SI) procedure [10] in conjunction with the perturbatively modified CAS (PMCAS-CI) reference functions obtained as linear combinations of all the states involved in the MS-CASPT2 calculation. Relativistic basis sets obtained from S(17s12p5d4f2g) C,N(14s9p4d3f) H(8s4p3d) primitive sets, the so called ANO-RCC basis sets [11], with the S[5s4p3d2f1g] C,N[4s3p2d1f] H[3s2p1d] contraction schemes were used throughout this Letter. The active space used in the calculations of CH3SNO comprises 14 electrons distributed in 11 orbitals (Scheme 1). The orbitals of the active space correspond to the occupied 2s(N), r(NO), p(NO), nr(NO), r(CS), r(SN), and np(S) orbitals, plus the unoccupied r⁄(NO), p⁄(NO), r⁄(CS), and r⁄(SN) orbitals. This active space is analogous to others used in several photodissocation studies of
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C. Ruano et al. / Chemical Physics Letters 553 (2012) 17–20
Scheme 1.
nitro- and nitrite-derivatives [12–17], where we demonstrated that it was the minimal active space to deal rigorously with reactions of nitro- and nitrite-derivatives. Otherwise, an active space of fewer dimensions could cause that the wave function of the system breaks symmetry as the molecule is distorted from its equilibrium position Scheme 2. Scheme 2.
3. Results and discussions The infrared study of methyl thionitrite in argon matrix [18] concluded that this molecule exists in two rotameric forms depending on the orientation of the in-plane CSNO atoms, i.e. syn- and anti-conformers. We have found that the syn-conformer presents in turn two stable conformations, which depend on the orientation of the in-plane hydrogen atom. These syn-conformers are almost isoenergetic. The main geometrical parameters of the Cs structures for the three conformers are collected in Table 1. The geometries have been optimized at the MS-CASPT2 level with a three state-average CAS-SCF reference wave function. In order to confirm that the respective geometries were true minima, the corresponding vibrational analysis was performed at the CAS-SCF level with the optimized geometry obtained at the CAS-SCF level as well. The determination of the Gibbs free energy difference of the syn-anti conversion from the infrared study was DrGo = 5.56 ± 0.76 kJ mol1 [18]. The value of DrGo (syn to anti-conformer reaction) computed by using the standard equations of statistical thermodynamics, in conjunction with the energetic and geometrical parameters calculated by ourselves (Table 1), amounts to 4.7 and 3.6 kJ mol1, for the syn (I) and syn (II) conformers, respectively. The Cs electronic configuration of the ground state of CH3SNO, with respect to the active space applied in this Letter, is:
Table 1 MS-CASPT2 geometrical parameters of syn- and anti-methyl thionitrite (Cs symmetry).a,b,c
a
Geometry
syn-CH3SNO (I)
syn-CH3SNO (II)
anti-CH3SNO
R(C–S) R(S–N) R(N–O) R(Ha–C)d R(Hb-C) A(CSN) A(SNO) A(HaCS) A(HbCS) Dh(CSNO) Dh(HaCSN) Dh(HbCSHa) E he
1.786 1.843 1.191 1.087 1.080 100.9 116.7 106.0 110.2 0.0 0.0 118.8 568.426 273 46
1.790 1.823 1.196 1.080 1.082 98.9 115.1 107.2 109.7 0.0 180.0 119.7 568.426 085 40
1.795 1.855 1.185 1.085 1.079 93.7 116.3 106.4 110.1 180.0 0.0 119.0 568.424 637 05
The reference active space comprises 14 electrons distributed in 11 orbitals. Two state average CAS-SCF reference wave function and 0.1 imaginary level shift. c Internuclear distance in Å, valence and dihedral angles in degrees. d In-plane hydrogen atom. e Electronic energy in hartrees. b
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C. Ruano et al. / Chemical Physics Letters 553 (2012) 17–20
½A0 ð2sN Þ2 rðNOÞ2 rðCSÞ2 rðSNÞ2 nr ðNOÞ2 r ðSNÞ0 r ðNOÞ0 r ðCSÞ0 00
2
2
0
½A pðNOÞ np ðSÞ p ðNOÞ
Transition
The vertical excitation energies of the low-lying excited states of the syn- and anti-rotamers are listed in Tables 2–4 respectively, along with the oscillator strength of each transition and the dominant electronic configuration. The results obtained for the three conformers are similar. The first excited state (S1) corresponds to an nrNO ? p⁄(NO) A00 transition and the second excited state (S2) is an npS ? p⁄(NO) A0 excitation. The oscillator strength of S1 is two orders of magnitude lower than that of S2, in accordance with the band intensities observed in the absorption spectrum. Excitations to S3, S4, and S5, which correspond to valence states as well, are doubly excitated in nature. The photodissociation mechanism of methyl thionitrite (synconformer I) leading to NO or SNO extrusion, respectively, have been studied by means of the interpolation vector method, a detailed discussion of this method can be found in reference 12–17. Figures 1 and 2 represent the potential energy curves for these dissociation reactions evaluated at the MS-CASPT2 level. The starting point on each curve corresponds to the geometry of the S0 energy minimum of CH3SNO and the final point to the dissociated fragments, CH3O (CH3) and NO (SNO), on their respective ground state minima and separated by a distance of 4.7 Å. The separation between the corresponding fragments at the final point, it is arbitrarily chosen, the only condition that is imposed is that the energy increment with respect to the previous point was less than 0.1 kcal/mol. There are several advantages of using the line interpolation vector method instead of a relaxed geometry scan: (i) the points obtained in the interpolation are perfectly ordered in a straight line; (ii) it is a relatively cheap calculation. In contrast, if we performed a more computationally expensive relaxed geometry scan, the obtained points will not be directly ordered in a straight line along the frozen coordinate. This a very important issue because two points of the scan considered as neighbors can occupy, in fact, very apart positions on the actual multidimensional potential energy surface. Another disadvantages related with the precedent issue is that it can be observed artificial energy barriers which, in turn, can induce a search of an inexistent transition state. The topology of the potential energy curves that lead to CH3S + NO formation (Figure 1) is understood in term of the nature of the salient fragments (CH3S and NO). NO is a doubly degenerate molecule on the ground state because the unpaired electron can occupy one of the two p⁄ equivalent orbitals. This degenerate state of NO splits into almost isoenergetic A0 and A00 states under the Cs symmetry of the CH3S + NO dissociation. On the other hand, the two lowest states of Cs methyl thionitrite are of A0 and A00 symmetry, depending on whether the unpaired electron occupies a p(A0 ) or p(A00 ) orbital. These states of CH3S are almost isoenergetic as
Table 2 MS-CASPT2 vertical transition energies of syn methyl thionitrite I (Cs symmetry).a,b,c Transition
Wave function
1A0 ? 1A00 , S1 1A0 ? 2A0 , S2
86% 73% 10% 88% 89% 74% 11%
1A0 ? 2A00 , S3 1A0 ? 3A0 , S4 1A0 ? 3A00 , S5 a b c d e
nrNO1 p⁄(NO)1 npS1p⁄(NO)1 nrNO1r⁄(SN)1 npS1r⁄(SN)1 nrNO0p⁄(NO)2 r(CS)1p⁄(NO)1 nr NO0 r⁄(SN)1p⁄(NO)1
Table 3 MS-CASPT2 vertical transition energies of syn methyl thionitrite II (Cs symmetry).a,b,c
Energy
d
f
e
2.43 3.59
4 104 3 102
5.26 6.00 6.24
2 103 4 104 1 102
The reference active space comprises 14 electrons distributed in 11 orbitals. Three state average CAS-SCF reference wave function. 0.1 imaginary level shift applied. Transition energy in eV. Oscillator strength.
0
00
1A ? 1A , S1 1A0 ? 2A0 , S2 1A0 ? 2A00 , S3 1A0 ? 3A0 , S4 1A0 ? 3A00 , S5 a b c d e
Wave function 1
⁄
1
89% nrNO p (NO) 73% npS1p⁄(NO)1 11% nrNO1r⁄(SN)1 89% np S1r⁄(SN)1 89% nrNO0p⁄(NO)2 73% r(CS)1p⁄(NO)1 8% nrNO0 r⁄(SN)1p⁄(NO)1
Energyd
fe
2.33 3.72
6 104 3 102
5.34 5.78 6.38
1 103 3 104 2 102
The reference active space comprises 14 electrons distributed in 11 orbitals. Three state average CAS-SCF reference wave function. 0.1 imaginary level shift applied. Transition energy in eV. Oscillator strength.
Table 4 MS-CASPT2 vertical transition energies of anti methyl thionitrite (Cs symmetry).a,b,c Transition 0
00
1A ? 1A , S1 1A0 ? 2A0 , S2 1A0 ? 2A00 , S3 1A0 ? 3A0 , S4 1A0 ? 3A00 , S5 a b c d e
Wave function 89% 72% 11% 89% 89% 56% 27%
1
⁄
1
nrNO p (NO) npS1p⁄(NO)1 nrNO1r⁄(SN)1 npS1r⁄(SN)1 nrNO0p⁄(NO)2 r(CS)]1p⁄(NO)1 nrNO0 r⁄(SN)1p⁄(NO)1
Energyd
fe
2.26 3.46
3 104 2 102
4.98 5.74 6.23
1 103 8 104 4 103
The reference active space comprises 14 electrons distributed in 11 orbitals. Three state average CAS-SCF reference wave function. 0.1 imaginary level shift applied. Transition energy in eV. Oscillator strength.
Figure 1. MS-CASPT2 potential energy curves for syn-CH3SNO dissociation into CH3S and NO radicals. Interatomic S–N distance in Angstroms in the lower X-axis scale.
well, being the A0 state the lower in energy. Therefore, as CH3SNO dissociates into CH3S + NO, it is found for the four lowest states of the product that they are almost degenerate. This is the resultant combination of the two lowest isoenergetic states of NO with the two lowest almost isoenergetic states of CH3S. At the dissociation point, 1A’(S0) corresponds to p⁄(NO) A0 + p(CH3S) A0 , 1A00 (S1) to p⁄(NO) A0 + p(CH3S) A00 , 2A0 (S2) to p⁄(NO) A00 + p(CH3S) A00 , and 2A’(S3) to p⁄(NO) A00 + p(CH3S) A0 . With respect to the dissociation curves that lead to dissociation of CH3SNO into CH3 + SNO (Figure 2), it must be remarked that there are several surface crossings along the dissociation paths. The role of these crossings would be relevant under excitation at short wavelength; however, this spectral region is beyond the
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with singly excited electronic configurations plus the S3 (nrp⁄) state with doubly excited configuration. The photodissociation reaction of methyl thionitrite has been studied along the two main reaction coordinates: (i) CH3S + NO and (ii) CH3 + SNO. It has been demonstrated that nitric oxide extrusion is the preferred process after excitation from both the dynamics and energetic points of view. Furthermore, the S–N bond rupture to lead NO is the lowest energy chemical process on the ground state, consequently, it is thermally the preferred process as well. Acknowledgments
Figure 2. MS-CASPT2 potential energy curves for syn-CH3SNO dissociation into CH3 and SNO radicals. Interatomic C–S distance in Angstroms in the lower X-axis scale.
This research has been supported by the Spanish Ministerio de Educación y Ciencia (Project CTQ2009-08549). The authors thank SCAI (University of Málaga) for economical support to update the MOLCAS software package. References
scopes of this Letter, provided that we have focused our attention on the excitation region used in the available experimental data. Some conclusions can be achieved from the inspection of these plots. First, there is no extra exit barrier for dissociation on the ground state in any of both processes, NO or SNO extrusions. Second, NO elimination from CH3SNO is 30 kcal/mol lower than SNO extrusion. Bond dissociation energies of CH3S–NO and CH3–SNO, calculated as the enthalpy of the corresponding dissociation reaction (syn conformer) amount to 30.0 kcal/mol and 56.7 kcal/mol, respectively. Third, the potential energy curves of the low-lying excited states (S1–S3) associated with nitric oxide formation are repulsive (Figure 1); in contrast, elimination of SNO on the same low-lying excited states are associative (Figure 2). Therefore, the preferred process for CH3SNO on the low-lying excited states is NO elimination. 4. Conclusions Methyl thionitrite has three valence states in the 2.0–6.0 eV energy range, which correspond to the S1 (nrp⁄) and S2 (npp⁄) states
[1] [2] [3] [4] [5] [6] [7]
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
D.L.H. Williams, Acc. Chem. Res. 32 (1999) 869. A.A. Eroy-Reveles, P.K. Mascharak, Med. Chem. 1 (2009) 1497. G.R. Kennedy, C.-L. Ning, J. Pfab, Chem. Phys. Lett. 292 (1998) 161. M.R.S. McCoustra, J. Pfab, Chem. Phys. Lett. 137 (1987) 355. H. Niki, P.D. Maker, C.M. Savage, L.P. Breitenbach, J. Phys. Chem. 87 (1983) 7. Q.K. Timerghazin, A.M. English, G.H. Peslherbe, Chem. Phys. Lett. 454 (2008) 24. B.O. Roos, in: K.P. Lawley (Ed.), Advances in Chemical Physics; Ab Initio Methods in Quantum Chemistry II, John Wiley & Sons, Chichester England, 1987, p. 399 (Chapter 69). J. Finley, P.-Å. Malmqvist, B.O. Roos, L. Serrano-Andrés, Chem. Phys. Lett. 288 (1998) 299. F. Aquilante et al., J. Comput. Chem. 31 (2010) 224. P.-Å. Malmqvist, Int. J. Quantum Chem. 30 (1986) 470. B.O. Roos, R. Lindh, P.-Å. Malmqvist, V. Veryazov, P.-O. Widmark, J. Phys. Chem. A 108 (2005) 2851. J.F. Arenas, J.C. Otero, D. Peláez, J. Soto, J. Chem. Phys. 119 (2003) 7814. J.F. Arenas, J.C. Otero, D. Peláez, J. Soto, J. Chem. Phys. 122 (2005) 084324. J. Soto, J.F. Arenas, J.C. Otero, D. Peláez, J. Phys. Chem. A 110 (2006) 8221. J.F. Arenas, F.J. Avila, J.C. Otero, D. Peláez, J. Soto, J. Phys. Chem. A 112 (2008) 249. J. Soto, D. Peláez, J.C. Otero, Phys. Chem. Chem. Phys. 11 (2009) 2631. J.F. Arenas, J.C. Otero, D. Peláez, J. Soto, J. Org. Chem. 71 (2006) 983. R.P. Müller, J. Robert Huber, J. Phys. Chem. 88 (1984) 1605.
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Multiconfigurational second-order perturbation study of the photochemical decomposition of methyl thionitrite Cristina Ruano, Juan C. Otero, Juan F. Arenas, Juan Soto ⇑ Department of Physical Chemistry, Faculty of Sciences, University of Málaga, E-29071 Málaga, Spain
a r t i c l e
i n f o
Article history: Received 13 June 2012 In final form 26 September 2012 Available online 12 October 2012
a b s t r a c t The photodissociation of methyl thionitrite (CH3SNO) has been computationally studied by means of CAS-SCF and MS-CASPT2 methods. The analysis of the multiconfigurational wavefunction corroborates the assignments of the absorption spectra, the visible and UV bands assigned to S0 ? S1(n, p⁄) and S0 ? S2(p, p⁄) transitions correspond to 1A0 ? 1A00 (nr, p⁄) and 1A0 ? 2A0 (np, p⁄) excitations, respectively. With respect to the photochemistry of CH3SNO, it is found that the potential energy surfaces associated with the low-lying excited states of this molecule (11A00 , 21A0 , and 21A00 ) are repulsive along the NO elimination coordinates. For this reason, population of such excited states leads to NO extrusion as the primary process, in agreement with experiments. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction S-nitrosothiols (RSNO’s) are biologically important because of their role in storing and transporting NO within the body and their potential medical use [1]. Recently, there is a growing interest in these compounds when they act as light-sensitive NO donor drugs, because they can deliver nitric oxide under the total control of irradiation at selected targets [2]. The photodissociation dynamics of the parent S-nitrosothiol molecule, methyl thionitrite (CH3SNO), has been previously studied [3–5]. The electronic absorption spectrum of methyl thionitrite shows a very weak band in the visible region around 500–600 nm and a stronger transition centered around 350 nm. The visible and UV bands are assigned to S0 ? S1(n, p⁄) and S0 ? S2(p, p⁄) transitions, respectively [3,4]. Irradiation of CH3SNO in the near-UV region yields CH3SSCH3 and NO, which points to an S–N bond cleavage [5] as the primary step according to the reaction: CH3SNO + hv ? CH3S + NO. Analysis of the NO photofragments after irradiation of CH3SNO at 450 [3] and 355 [4] nm has shown that the populated state at these wavelengths corresponds to a p ? p⁄ transition on a repulsive potential energy surface [3]. The multireference character of the wavefunction of S-nitrothiols was recently demonstrated by an ab initio study on HSNO [6] performed with the complete active space self-consistent field (CAS-SCF) method. We report now the multiconfigurational calculations of the electronic excitations and the dissociation potential energy curves for methyl thionitrite. It will be shown that S1, S2, and S3 states of CH3SNO correspond to repulsive surfaces with respect to NO elimination. Additionally, the assignment of the ⇑ Corresponding author. E-mail address: [email protected] (J. Soto). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.09.060
observed spectrum is supported by means of transition dipole moment calculations and analysis of the wave function. Furthermore, this Letter is of potential interest as a benchmark for calculations on S-nitrosothiols derivatives performed with other theoretical approximations. 2. Computational details Calculations have been performed with the complete active space self-consistent field (CAS-SCF) method and the multistate extension of the multiconfigurational second-order perturbation (MS-CASPT2) approximation [7,8] as they are implemented in the MOLCAS 7.6 program [9]. Geometry optimizations followed two strategies: (i) optimization at the CAS-SCF level of theory by computation of analytical energy gradients and (ii) optimizations with the MS-CASPT2 method by computation of numerical energy gradients. Two state average CAS-SCF reference wave functions were used in the MS-CASPT2 optimizations. Transition dipole moments were computed according to the CAS state interaction (CAS-SI) procedure [10] in conjunction with the perturbatively modified CAS (PMCAS-CI) reference functions obtained as linear combinations of all the states involved in the MS-CASPT2 calculation. Relativistic basis sets obtained from S(17s12p5d4f2g) C,N(14s9p4d3f) H(8s4p3d) primitive sets, the so called ANO-RCC basis sets [11], with the S[5s4p3d2f1g] C,N[4s3p2d1f] H[3s2p1d] contraction schemes were used throughout this Letter. The active space used in the calculations of CH3SNO comprises 14 electrons distributed in 11 orbitals (Scheme 1). The orbitals of the active space correspond to the occupied 2s(N), r(NO), p(NO), nr(NO), r(CS), r(SN), and np(S) orbitals, plus the unoccupied r⁄(NO), p⁄(NO), r⁄(CS), and r⁄(SN) orbitals. This active space is analogous to others used in several photodissocation studies of
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C. Ruano et al. / Chemical Physics Letters 553 (2012) 17–20
Scheme 1.
nitro- and nitrite-derivatives [12–17], where we demonstrated that it was the minimal active space to deal rigorously with reactions of nitro- and nitrite-derivatives. Otherwise, an active space of fewer dimensions could cause that the wave function of the system breaks symmetry as the molecule is distorted from its equilibrium position Scheme 2. Scheme 2.
3. Results and discussions The infrared study of methyl thionitrite in argon matrix [18] concluded that this molecule exists in two rotameric forms depending on the orientation of the in-plane CSNO atoms, i.e. syn- and anti-conformers. We have found that the syn-conformer presents in turn two stable conformations, which depend on the orientation of the in-plane hydrogen atom. These syn-conformers are almost isoenergetic. The main geometrical parameters of the Cs structures for the three conformers are collected in Table 1. The geometries have been optimized at the MS-CASPT2 level with a three state-average CAS-SCF reference wave function. In order to confirm that the respective geometries were true minima, the corresponding vibrational analysis was performed at the CAS-SCF level with the optimized geometry obtained at the CAS-SCF level as well. The determination of the Gibbs free energy difference of the syn-anti conversion from the infrared study was DrGo = 5.56 ± 0.76 kJ mol1 [18]. The value of DrGo (syn to anti-conformer reaction) computed by using the standard equations of statistical thermodynamics, in conjunction with the energetic and geometrical parameters calculated by ourselves (Table 1), amounts to 4.7 and 3.6 kJ mol1, for the syn (I) and syn (II) conformers, respectively. The Cs electronic configuration of the ground state of CH3SNO, with respect to the active space applied in this Letter, is:
Table 1 MS-CASPT2 geometrical parameters of syn- and anti-methyl thionitrite (Cs symmetry).a,b,c
a
Geometry
syn-CH3SNO (I)
syn-CH3SNO (II)
anti-CH3SNO
R(C–S) R(S–N) R(N–O) R(Ha–C)d R(Hb-C) A(CSN) A(SNO) A(HaCS) A(HbCS) Dh(CSNO) Dh(HaCSN) Dh(HbCSHa) E he
1.786 1.843 1.191 1.087 1.080 100.9 116.7 106.0 110.2 0.0 0.0 118.8 568.426 273 46
1.790 1.823 1.196 1.080 1.082 98.9 115.1 107.2 109.7 0.0 180.0 119.7 568.426 085 40
1.795 1.855 1.185 1.085 1.079 93.7 116.3 106.4 110.1 180.0 0.0 119.0 568.424 637 05
The reference active space comprises 14 electrons distributed in 11 orbitals. Two state average CAS-SCF reference wave function and 0.1 imaginary level shift. c Internuclear distance in Å, valence and dihedral angles in degrees. d In-plane hydrogen atom. e Electronic energy in hartrees. b
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½A0 ð2sN Þ2 rðNOÞ2 rðCSÞ2 rðSNÞ2 nr ðNOÞ2 r ðSNÞ0 r ðNOÞ0 r ðCSÞ0 00
2
2
0
½A pðNOÞ np ðSÞ p ðNOÞ
Transition
The vertical excitation energies of the low-lying excited states of the syn- and anti-rotamers are listed in Tables 2–4 respectively, along with the oscillator strength of each transition and the dominant electronic configuration. The results obtained for the three conformers are similar. The first excited state (S1) corresponds to an nrNO ? p⁄(NO) A00 transition and the second excited state (S2) is an npS ? p⁄(NO) A0 excitation. The oscillator strength of S1 is two orders of magnitude lower than that of S2, in accordance with the band intensities observed in the absorption spectrum. Excitations to S3, S4, and S5, which correspond to valence states as well, are doubly excitated in nature. The photodissociation mechanism of methyl thionitrite (synconformer I) leading to NO or SNO extrusion, respectively, have been studied by means of the interpolation vector method, a detailed discussion of this method can be found in reference 12–17. Figures 1 and 2 represent the potential energy curves for these dissociation reactions evaluated at the MS-CASPT2 level. The starting point on each curve corresponds to the geometry of the S0 energy minimum of CH3SNO and the final point to the dissociated fragments, CH3O (CH3) and NO (SNO), on their respective ground state minima and separated by a distance of 4.7 Å. The separation between the corresponding fragments at the final point, it is arbitrarily chosen, the only condition that is imposed is that the energy increment with respect to the previous point was less than 0.1 kcal/mol. There are several advantages of using the line interpolation vector method instead of a relaxed geometry scan: (i) the points obtained in the interpolation are perfectly ordered in a straight line; (ii) it is a relatively cheap calculation. In contrast, if we performed a more computationally expensive relaxed geometry scan, the obtained points will not be directly ordered in a straight line along the frozen coordinate. This a very important issue because two points of the scan considered as neighbors can occupy, in fact, very apart positions on the actual multidimensional potential energy surface. Another disadvantages related with the precedent issue is that it can be observed artificial energy barriers which, in turn, can induce a search of an inexistent transition state. The topology of the potential energy curves that lead to CH3S + NO formation (Figure 1) is understood in term of the nature of the salient fragments (CH3S and NO). NO is a doubly degenerate molecule on the ground state because the unpaired electron can occupy one of the two p⁄ equivalent orbitals. This degenerate state of NO splits into almost isoenergetic A0 and A00 states under the Cs symmetry of the CH3S + NO dissociation. On the other hand, the two lowest states of Cs methyl thionitrite are of A0 and A00 symmetry, depending on whether the unpaired electron occupies a p(A0 ) or p(A00 ) orbital. These states of CH3S are almost isoenergetic as
Table 2 MS-CASPT2 vertical transition energies of syn methyl thionitrite I (Cs symmetry).a,b,c Transition
Wave function
1A0 ? 1A00 , S1 1A0 ? 2A0 , S2
86% 73% 10% 88% 89% 74% 11%
1A0 ? 2A00 , S3 1A0 ? 3A0 , S4 1A0 ? 3A00 , S5 a b c d e
nrNO1 p⁄(NO)1 npS1p⁄(NO)1 nrNO1r⁄(SN)1 npS1r⁄(SN)1 nrNO0p⁄(NO)2 r(CS)1p⁄(NO)1 nr NO0 r⁄(SN)1p⁄(NO)1
Table 3 MS-CASPT2 vertical transition energies of syn methyl thionitrite II (Cs symmetry).a,b,c
Energy
d
f
e
2.43 3.59
4 104 3 102
5.26 6.00 6.24
2 103 4 104 1 102
The reference active space comprises 14 electrons distributed in 11 orbitals. Three state average CAS-SCF reference wave function. 0.1 imaginary level shift applied. Transition energy in eV. Oscillator strength.
0
00
1A ? 1A , S1 1A0 ? 2A0 , S2 1A0 ? 2A00 , S3 1A0 ? 3A0 , S4 1A0 ? 3A00 , S5 a b c d e
Wave function 1
⁄
1
89% nrNO p (NO) 73% npS1p⁄(NO)1 11% nrNO1r⁄(SN)1 89% np S1r⁄(SN)1 89% nrNO0p⁄(NO)2 73% r(CS)1p⁄(NO)1 8% nrNO0 r⁄(SN)1p⁄(NO)1
Energyd
fe
2.33 3.72
6 104 3 102
5.34 5.78 6.38
1 103 3 104 2 102
The reference active space comprises 14 electrons distributed in 11 orbitals. Three state average CAS-SCF reference wave function. 0.1 imaginary level shift applied. Transition energy in eV. Oscillator strength.
Table 4 MS-CASPT2 vertical transition energies of anti methyl thionitrite (Cs symmetry).a,b,c Transition 0
00
1A ? 1A , S1 1A0 ? 2A0 , S2 1A0 ? 2A00 , S3 1A0 ? 3A0 , S4 1A0 ? 3A00 , S5 a b c d e
Wave function 89% 72% 11% 89% 89% 56% 27%
1
⁄
1
nrNO p (NO) npS1p⁄(NO)1 nrNO1r⁄(SN)1 npS1r⁄(SN)1 nrNO0p⁄(NO)2 r(CS)]1p⁄(NO)1 nrNO0 r⁄(SN)1p⁄(NO)1
Energyd
fe
2.26 3.46
3 104 2 102
4.98 5.74 6.23
1 103 8 104 4 103
The reference active space comprises 14 electrons distributed in 11 orbitals. Three state average CAS-SCF reference wave function. 0.1 imaginary level shift applied. Transition energy in eV. Oscillator strength.
Figure 1. MS-CASPT2 potential energy curves for syn-CH3SNO dissociation into CH3S and NO radicals. Interatomic S–N distance in Angstroms in the lower X-axis scale.
well, being the A0 state the lower in energy. Therefore, as CH3SNO dissociates into CH3S + NO, it is found for the four lowest states of the product that they are almost degenerate. This is the resultant combination of the two lowest isoenergetic states of NO with the two lowest almost isoenergetic states of CH3S. At the dissociation point, 1A’(S0) corresponds to p⁄(NO) A0 + p(CH3S) A0 , 1A00 (S1) to p⁄(NO) A0 + p(CH3S) A00 , 2A0 (S2) to p⁄(NO) A00 + p(CH3S) A00 , and 2A’(S3) to p⁄(NO) A00 + p(CH3S) A0 . With respect to the dissociation curves that lead to dissociation of CH3SNO into CH3 + SNO (Figure 2), it must be remarked that there are several surface crossings along the dissociation paths. The role of these crossings would be relevant under excitation at short wavelength; however, this spectral region is beyond the
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C. Ruano et al. / Chemical Physics Letters 553 (2012) 17–20
with singly excited electronic configurations plus the S3 (nrp⁄) state with doubly excited configuration. The photodissociation reaction of methyl thionitrite has been studied along the two main reaction coordinates: (i) CH3S + NO and (ii) CH3 + SNO. It has been demonstrated that nitric oxide extrusion is the preferred process after excitation from both the dynamics and energetic points of view. Furthermore, the S–N bond rupture to lead NO is the lowest energy chemical process on the ground state, consequently, it is thermally the preferred process as well. Acknowledgments
Figure 2. MS-CASPT2 potential energy curves for syn-CH3SNO dissociation into CH3 and SNO radicals. Interatomic C–S distance in Angstroms in the lower X-axis scale.
This research has been supported by the Spanish Ministerio de Educación y Ciencia (Project CTQ2009-08549). The authors thank SCAI (University of Málaga) for economical support to update the MOLCAS software package. References
scopes of this Letter, provided that we have focused our attention on the excitation region used in the available experimental data. Some conclusions can be achieved from the inspection of these plots. First, there is no extra exit barrier for dissociation on the ground state in any of both processes, NO or SNO extrusions. Second, NO elimination from CH3SNO is 30 kcal/mol lower than SNO extrusion. Bond dissociation energies of CH3S–NO and CH3–SNO, calculated as the enthalpy of the corresponding dissociation reaction (syn conformer) amount to 30.0 kcal/mol and 56.7 kcal/mol, respectively. Third, the potential energy curves of the low-lying excited states (S1–S3) associated with nitric oxide formation are repulsive (Figure 1); in contrast, elimination of SNO on the same low-lying excited states are associative (Figure 2). Therefore, the preferred process for CH3SNO on the low-lying excited states is NO elimination. 4. Conclusions Methyl thionitrite has three valence states in the 2.0–6.0 eV energy range, which correspond to the S1 (nrp⁄) and S2 (npp⁄) states
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