J. Phys. Chem. A 2010, 114, 3703–3712
3703
Conformational Behavior of CH3OC(O)SX (X ) CN and SCN) Pseudohalide Congeners. A Combined Experimental and Theoretical Study Sonia Torrico-Vallejos,† Mauricio F. Erben,*,† Mao-Fa Ge,‡ Helge Willner,§ and Carlos O. Della Ve´dova*,†,| CEQUINOR (CONICET-UNLP), Departamento de Quı´mica, Facultad de Ciencias Exactas, UniVersidad Nacional de La Plata, C.C. 962, 47 esq. 115, La Plata (B1900AJL), Buenos Aires, Repu´blica Argentina, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China, Fachbereich C-Anorganische Chemie, Bergische UniVersita¨t, Wuppertal Gausstrasse 20, 47097 Wuppertal, Germany, and Laboratorio de SerVicios a la Industria y al Sistema Cientı´fico (LaSeISiC) (UNLP-CIC-CONICET), Camino Centenario e/505 y 508, (1903) Gonnet, Repu´blica Argentina ReceiVed: December 21, 2009; ReVised Manuscript ReceiVed: February 1, 2010
Pure methoxycarbonylsulfenyl cyanide, CH3OC(O)SCN (I), and methoxycarbonylsulfenyl thiocyanate, CH3OC(O)SSCN (II), were prepared by reacting liquid CH3OC(O)SCl with either AgCN or AgSCN, respectively. Compounds I and II were characterized by 1H NMR, CG-MS, and vibrational (FTIR and FTRaman) techniques. The conformational properties have been studied by using vibrational spectroscopy [infrared (gaseous, liquid, and Ar matrix isolated), Raman (liquid) spectroscopy] together with quantum chemical calculations at the B3LYP and MP2 methods with the extended 6-311++G** and aug-cc-pVTZ basis sets. Compound I exhibits a conformational equilibrium at room temperature having the most stable form Cs symmetry with a synperiplanar (syn) orientation of the carbonyl double bond (CdO) with respect to both the CH3O- and -SCN groups (syn-syn). Several bands assigned to a second conformer have been observed in the IR matrix spectra. This rotamer presents an antiperiplanar orientation of the thiocyanate group (syn-anti). Evaluating the equilibrium compositions at different temperatures by quenching the gas phase mixtures as Ar matrices allowed us to determine the conformational enthalpy difference ∆H0 ) H0(syn-anti) - H0(syn-syn) ) 0.80(18) kcal mol-1. A similar conformational behavior has been determined for compound II. Thermodynamic properties were also computed at the high-level G2MP2 and G3 model chemistry methods. The importance of mesomeric (resonance) and anomeric (hyperconjugation) electronic interaction in the conformational behavior is evaluated by using the NBO approach for both species. Introduction Recently, we reported the synthesis of the novel compounds CH3OC(O)SNCO1 and CH3OC(O)SSCF32 derived from CH3OC(O)SCl. A complete structural study was conducted mainly by focusing in the conformational behavior around the sulfenylcarbonyl moiety [-C(O)S-]. The presence of two conformers could be clearly determined at room temperature, the syn form (the CdO double bond in synperiplanar orientation with respect to the S-NCO and S-S bonds, respectively) being the preferred conformation, with an important contribution of the anti form, of 19(5)% in the case of CH3OC(O)SSCF3. Moreover, the isocyanate species was found to be a very versatile reagent for the synthesis of carbamates3 and N,N′disubstituted ureas.4 Thus, following this work, we became interested in the two thiocyanate derivatives CH3OC(O)SCN and CH3OC(O)SSCN, which offer the possibility of comparing the effect that pseudohalide substituents exert on the CH3OC(O)S- group. * To whom correspondence should be addressed. Tel/Fax: +54-221425-9485. E-mail:
[email protected] (M.F.E.) and carlosdv@ quimica.unlp.edu.ar (C.O.D.V.). † Universidad Nacional de La Plata. ‡ Chinese Academy of Sciences. § Bergische Universita¨t. | Laboratorio de Servicios a la Industria y al Sistema Cientı´fico.
Since thiocyanates (R-SCN) are important reagents for the preparation of sulfur-containing organic compounds, they are highlighted in several reviews.5–8 To introduce the thiocyanate group into an organic molecule, a convenient synthetic route involves the reaction of metal thiocyanates with organic halides. The thiocyanate group is thermally unstable and simple chromatography or a prolonged heating over 50 °C can cause an intramolecular rearrangement to the thermodynamically favored isothiocyanate isomers.9 This interconversion between these linkage isomers has been known since 1873 when Gerlich10 and Billeter11 independently observed that allyl thiocyanate isomerizes during distillation. Correspondingly, the reaction between CH3OC(O)Cl with KSCN yields mainly the isothiocyanate derivative [CH3OC(O)NCS], with only a minor contribution of CH3OC(O)SCN, as reported by Liotta and Engel.12 Thiocyanate compounds with an SCN group attached directly to the sulfur atom, XSSCN, are well-known. The first thiocyanate containing a formal single S-S bond, NCSSCN, was synthesized by So¨derba¨ck in 1918.13 Halogenated derivatives, such as FC(O)SSCN and ClC(O)SSCN, were also reported by Haas14,15 and Jochims.16 However, these compounds received little attention until recently, when the electronic properties of XC(O)SSCN (X ) F17 and CH3O-18) were studied by photoelectron spectroscopy and quantum chemical calculations. Both compounds were obtained in situ by the heterogeneous reaction
10.1021/jp912044r 2010 American Chemical Society Published on Web 02/18/2010
3704
J. Phys. Chem. A, Vol. 114, No. 10, 2010
Torrico-Vallejos et al.
SCHEME 1: Representation of the Conformers of CH3OC(O)SCN
between solid AgCN and vapors of the corresponding sulfenyl chloride [XC(O)SCl, X ) F and CH3O]. In this work, the synthesis, isolation, and characterization of two pseudohalide derivatives, i.e., CH3OC(O)SCN (I) and CH3OC(O)SSCN (II), are presented. Gas and liquid phase infrared measurements as well as extensive quantum chemical calculations have been performed. For CH3OC(O)SCN, Armatrix IR spectra have been also recorded at different temperatures of the deposition nozzle to evaluate possible conformer mixtures in the gas phase. The vibrational analysis is completed with the Raman spectra of both compounds in the liquid phase. Thus, the structural and vibro-conformational properties were determined for both molecules. Finally, the NBO population analyses were applied to rationalize the effect of the electronic interactions on these properties. Results and Discussion Quantum Chemical Calculations. To the best of our knowledge theoretical studies on the CH3OC(O)SCN molecule have not been reported. In principle, the compound may be present in at least four conformers with a planar skeleton, depending on the orientation around the OsC and CsS single bonds (see Scheme 1). Taking into consideration structural studies previously reported for the methoxycarbonyl CH3OC(O)- moiety,19–25 a syn orientation of the δ(COsCdO) dihedral angle is preferred, the anti form being higher in energy. The conformational properties of sulfenylcarbonyl compounds, with the general formula XC(O)SY, have been studied and the preference of a synperiplanar conformation around the C-S single bond was well-established.19,26–28 Furthermore, in a few examples, it has been experimentally established that the antiperiplanar conformation appears as a second stable form at ambient temperature.29,30 Therefore, the potential energy function for internal rotation around the δ(OdCsSC) dihedral angle was calculated at the B3LYP/6-31G* level by allowing geometry optimizations with the dihedral angle varying from 0 to 180° in steps of 30°. The potential energy curve is shown in Figure 1. Two structures correspond to minima in the potential energy curve, corresponding to the syn-syn and syn-anti forms shown in Scheme 1. The most stable one displays a syn orientation of the CdO double bond with respect to the S-C bond [δ(OdCsSC) ) 0°] while the anti form (syn-anti), with δ(OdCsSC) ) 180°, is higher in energy by only ca. 0.4 kcal mol-1. Subsequently, full geometry optimizations and frequency calculations were performed for each of the most stable structures with the B3LYP and MP2 methods and the 6-31G*, 6-311++G**, and aug-cc-pVTZ basis sets. Predicted relative energies, ∆E° (corrected by zero-point energy) are listed in Table 1. The two computational methods predict the structure with syn orientation around both O-C and C-S single bonds to be the most stable conformer of CH3OC(O)SCN. The second stable form, higher in energy by 0.31 (B3LYP/6-311++G**) or 0.56
Figure 1. Calculated potential function (B3LYP/6-31G*) for internal rotation around the C-S single bond in CH3OC(O)SCN.
kcal mol-1 (MP2/6-311++G**) corresponds to a conformer with anti orientation of the δ(OdCsSC) dihedral angle. A third form, the anti-syn conformer in Scheme 1, was found to be a stable conformer with calculated energies higher than 6 kcal mol-1 with respect to the minimum (see Table 1) and is not expected to be detected under the conditions used in our experiments. On the other hand, the anti-anti rotamer does not correspond to a minimum, probably because of steric repulsions between the methyl and thiocyanate groups. Figure 2 shows the molecular models for the two main forms of CH3OC(O)SCN. For compound II, a theoretical study at the B3PW91/6-31+G* level of approximation was recently reported by Du et al.18 The main objective was the assignment of the photoelectron spectrum through the determination of molecular orbital characters. Interestingly, two conformations were found to be very close in energy. The most stable conformer has synperiplanar orientation of both O-C and S-S single bonds with respect to the CdO double bond (syn-syn), while the corresponding form having the CdO and S-S bonds in mutual antiperiplanar orientation (syn-anti) lies only 0.81 kcal mol-1 higher in energy (B3LYP/6-311G*) than the most stable form. The PES was assigned on the basis of the exclusive presence of the most stable conformer. From the conformational point of view, such a small energy difference between both conformers of compound II is of prime interest.31–34 Thus, full geometry optimizations and frequency calculations were performed for each of the most stable structures of CH3OC(O)SSCN with the B3LYP and the secondorder perturbation theory methods and the 6-31G* and 6-311++G** basis sets. Our results are in perfect agreement with those obtained recently by Ge et al.18 One of the important structural features of this compound is the dihedral angle around the S-S bond, with a computed value of 87.4° at the B3LYP/ 6-311++G** level of approximation. Predicted relative energies, ∆E° (corrected by zero-point energy) are listed in Table S1, in the Supporting Information. Vibrational Analysis of CH3OC(O)SCN. The vibrational and conformational properties of CH3OC(O)NCS were studied by Campbell et al.35 On the other hand, due to the inherent instability and the difficulties to isolate the thiocyanate, little is known about the linkage isomer CH3OC(O)SCN. The IR (liquid) and Raman (liquid) spectra of CH3OC(O)SCN are shown in Figure 3. A tentative assignment of the observed bands was carried out by comparison with calculated wave-
Conformational Behavior of CH3OC(O)SX
J. Phys. Chem. A, Vol. 114, No. 10, 2010 3705
TABLE 1: Calculated Relative Energies Corrected by Zero-Point Energy (kcal mol-1) and Vibrational Frequency of the CdO Stretching Mode (cm-1) with IR Intensities (km mol-1, in Parentheses) for Three Conformers of CH3OC(O)SCN syn-syn method of calculation B3LYP/6-31G* B3LYP/6-311++G** B3LYP/aug-cc-pVTZ MP2/6-31G* MP2/6-311++G** MP2/aug-cc-pVTZ G2MP2 G3 CCSD(T)/6-311++G**
∆E° a
0.00 0.00b 0.00c 0.00d 0.00e 0.00f 0.00g 0.00h 0.00i
syn-anti
anti-syn
ν(CdO)
∆E°
ν(CdO)
∆E°
ν(CdO)
1870(221) 1840(272) 1829(249) 1848(180) 1824(225)
0.42 0.31 0.24 0.70 0.56 0.44 0.37 0.39 0.54
1849(326) 1813(421) 1801(387) 1831(267) 1801(351)
6.64 6.68
1899(315) 1872(380)
7.48
1865(275)
a E° ) -719.421522 hartree. b E° ) -719.547432 hartree. c E° ) -719.588356 hartree. d E° ) -717.969509 hartree. e E° ) -718.171574 hartree. f E ) -718.4966263 hartree, zero-point correction at the MP2/6-311++G** level. g G2MP2 energy ) -718.564018 hartree. h G3 energy ) -719.159740 hartree. i E ) -718.3057792 hartree, optimized geometry and zero-point correction at the MP2/6-311++G** level.
Conformational Equilibrium
Figure 2. Molecular models for the two main conformers of CH3OC(O)SCN.
numbers and on comparison with spectra of related molecules, specially XC(O)SNCO (X ) Cl, F and CH3O)14,36,27 and XSCN (X ) H, Cl, Br).37 Furthermore, the potential energy distribution (PED) associated with each normal vibrational mode was calculated under the harmonic assumption. Experimental and calculated [MP2/6-311++G**] frequencies, intensities, and their tentative assignments are given in Table 2. This table also lists the data obtained from the isolated Ar-matrix infrared spectrum. The characteristic mode of vibration of thiocyanate compounds corresponds to a band with weak intensity located at 2176 cm-1 assigned to the ν(SCtN) stretching mode in the liquid-phase IR spectrum, with its counterpart at 2178 cm-1 in the liquid Raman spectrum as the most intense signal.38,39 This fundamental is absent in the gas-phase spectrum and is observed as a very low intensity absorption in the Ar-matrix spectrum. Characteristic absorptions corresponding to the stretching mode of vibrations of OCH3 and CdO groups appear at 2970 and 1794/1765 cm-1, respectively. Good agreement is obtained when the experimental vibrational frequencies and the calculated values are compared. From the PED analysis, the characteristic fundamentals are clearly established, but many other vibrations are mixed. It becomes apparent that absorptions observed in the gas IR spectrum and in the Ar-matrix spectrum can be assigned, in principle, to the presence of a second conformer of CH3OC(O)SCN at room temperature. This topic will be investigated in more detail in the following section.
The occurrence of two bands in the region of the CdO group in the IR (gaseous an Ar matrix isolated) spectrum (Figure 4) suggests that an equilibrium of two conformers in similar concentrations is present at room temperature. It is well-known that the ν(CdO) normal mode of carbonyl compounds is very sensitive to conformational properties.35,40 The carbonyl stretching region (1900-1700 cm-1) of the infrared spectra of CH3OC(O)SCN in the vapor phase and isolated in an Ar-matrix are shown in detail in Figure 4. Two intense absorptions are evident; one band in the gaseous spectrum occurs at 1794 cm-1 and the other at 1765 cm-1. The well-defined vibrationalrotational contour of the blue-shifted band allows assigning this absorption to the syn-syn form in a straightforward manner. The almost parallel orientation of the carbonyl oscillator (dipole derivative vector) with respect to the principal axis of inertia B, a B-type band is expected for the ν(CdO) normal mode of the syn-syn form. In the same way, from the band contour of the absorption at 1765 cm-1, the syn-anti form is proposed. Because of the orientation of the carbonyl oscillator with respect to the principal axis of inertia A and B, an AB hybrid band type is expected for the ν(CdO) normal mode of the syn-anti conformer. The inset of Figure 4 shows the principal axis of inertia for both conformers. Following the classical procedure proposed by Seth-Paul,41 a semiquantitative analysis of the band contours expected for both prolate asymmetric tops syn-syn and syn-anti rotamers, has been further performed.42 The observed P-R separations for the B-type band at 1975 cm-1 (10 cm-1) and for the ABtype band at 1765 cm-1 (11 cm-1) are good reproduced by the values calculated for the syn and anti forms of CH3OC(O)SCN whose values are 9 and 11 cm-1, respectively. The detailed procedure for the calculation of P-R separation is given as Supporting Information. Additionally, the calculated wavenumber difference [B3LYP/ 6-311++G**] between the CdO stretching modes of the two conformers is 27 cm-1 (Table 1), in good agreement with the experimentally observed value of 29 cm-1 in both the Ar-matrix and gas-phase spectra. This correspondence is observed in Figure 4, where the simulated spectrum from the MP2/6-311++G** calculated frequencies is shown, assuming a mixture of the syn-syn and syn-anti forms in a 75:25 ratio. Variable Temperature Conformational Equilibria Quenched by Matrix Isolation Taking into account the experimental evidence coming from the gas-phase infrared spectrum about the presence of more than
3706
J. Phys. Chem. A, Vol. 114, No. 10, 2010
Torrico-Vallejos et al.
Figure 3. Liquid IR (top) and liquid Raman (bottom) spectra of CH3OC(O)SCN.
one conformer, and that rich conformational equilibria are envisaged by the quantum chemical calculations, further experimental evidence is desirable to fully characterize the conformational properties of compound I. If the barrier between the rotamers is
