G3 Model Chemistry Research Articles

Raman and infrared spectra, quantum chemical calculations, conformations and spectral assignments of 1-chloro-1-methyl-1-silacyclohexane

A 1,1-disubstituted silacyclohexane, 1-chloro-1-methyl-1-silacyclohexane, C5H10SiClCH3, was synthesized and studied by infrared and Raman spectroscopy. The infrared spectra of the vapor and liquid were recorded at ambient temperature and the solid sample investigated at 78K. Negligible spectral changes ocurred at 78K compared with the fluid state, but after annealing to ca. 165K an apparent crystal was formed.Raman spectra of the liquid were recorded at 293K and depolarization data obtained. Additional Raman spectra were recorded at various temperatures between 293 and 143K. A supercooled liquid appeared after slow cooling, but an amorphous solid phase was observed after shock freezing to 78K. After annealing, a plastic phase was observed and an anisotropic crystal appeared after further annealing. In the crystalline phase spectral shifts and some vanishing bands were observed and only the a (Cl) conformer remained.The compound exists in two conformers, equatorial (Cl) (e) and axial (Cl)(a) in the liquid, amorphous and plastic phases, but only the a-conformer was present in the crystal. The experimental results suggest that the a-conformer has 1.5±0.5kJmol−1 lower enthalpy than e in the liquid.B3LYP calculations with various basis sets and the G3 model chemistry gave conformational energy difference ΔE (a–e) in the range 3.2–2.4kJmol−1. Infrared and Raman intensities, polarization ratios and vibrational frequencies for the two conformers were calculated. The 32A′ and 25A″ modes for the e(Cl) and a(Cl) conformations were assigned, supported by anharmonic B3LYP/cc-pVTZ calculations.

Infrared and Raman spectra, conformations, quantum chemical calculations and spectral assignments of 1-methyl-1-silacyclohexane

Raman spectra of of 1-methyl-1-silacyclohexane as a liquid were recorded at 293K and depolarization data obtained. Additional Raman spectra were recorded at various temperatures between 293 and 143K, and intensity changes with temperature of certain Raman bands were detected. A supercooled liquid appeared after slow cooling, but an amorphous phase was observed after shock freezing to 78K. After annealing, first a plastic phase and subsequently a crystal were observed. In the crystalline phase spectral shifts and some vanishing bands were observed.The infrared spectra of the vapor and liquid were studied at ambient temperature and the solid sample investigated at 78K. Negligible spectral changes ocurred at 78K compared with the fluid state, but after annealing to ca. 170K an apparent crystal was formed and a few bands vanished and others were shifted.The compound exists in two conformers, equatorial (e) and axial (a) in the liquid, amorphous and plastic phases, but only the e-conformer was present in the crystal. The experimental results suggest that the e-conformer has 0.6kJmol−1 lower energy than a in the liquid.B3LYP calculations with various basis sets gave a conformational enthalpy difference ΔH (a–e) around 2.4kJmol−1 while G3 model chemistry gave 0.6kJmol−1. Infrared and Raman intensities, polarization ratios and vibrational frequencies for the e and a conformers were calculated. The fundamental wavenumbers were also derived in the anharmonic approximation in B3LYP/cc-pVTZ calculations. A relative deviation of 0.94% and 1.31% between the observed and calculated wave numbers for the 57 modes of the e-and a-conformers, respectively, was obtained.

Vibrational spectra, conformations, quantum chemical calculations and spectral assignments of 1-chloro-1-silacyclohexane

The infrared spectra of 1-chloro-1-silacyclohexane have been studied as a vapour and liquid at ambient temperature and as amorphous and annealed crystalline solids at 78K. Various infrared bands present in the vapour and liquid states vanished in the crystalline state upon cooling. Raman spectra of the liquid were recorded at 293K and polarization data obtained. Additional Raman spectra were recorded at various temperatures between 293 and 163K, and intensity changes with temperature of certain Raman bands were detected. A crystalline phase was observed around 140K, leading to spectral shifts and a number of vanishing bands.The compound exists in two conformers, equatorial (e) and axial (a) in the fluid phases, but only the a-conformer was present in the crystal. The experimental results suggest that the a-conformer has 2.8kJmol−1 lower enthalpy than e in the liquid, leading to 69% of the a conformer at 293K.B3LYP calculations with various basis sets and the G3 model chemistry gave conformational energy difference ΔE (e−a) in the range 0.4–1.4kJmol−1. Infrared and Raman intensities, polarization ratios and vibrational frequencies for the e and a conformers were calculated. The fundamental wavenumbers were also derived in the anharmonic approximation in B3LYP/cc-pVTZ calculations, a relative deviation of less than 2% between the observed and calculated wave numbers for the 48 modes of the e- and a-conformers was obtained.

Infrared and Raman spectra, conformations, ab initio calculations and spectral assignments of 1-fluoro-1-silacyclohexane

Raman spectra of 1-fluoro-1-silacyclohexane as a liquid were recorded at 293 K and polarization data obtained. Additional Raman spectra were recorded at various temperatures between 293 and 143 K, and intensity changes of certain bands with temperature were investigated. An apparently plastic phase was observed around 170 K, but no definite crystallization was ever obtained on cooling. The infrared spectra have been studied of the vapor, of an amorphous solid at 78 K and of the liquid in the range 600–100 cm−1. No infrared bands present in the vapor or liquid vanished upon cooling. The compound exists a priori in two conformers, equatorial (e) and axial (a), and the experimental results suggest an equilibrium in which the a-conformer has 1.2 kJ mol−1 lower enthalpy than the e-conformer in the liquid, leading to 60% a-conformer at ambient temperature. B3LYP calculations with various basis sets and the G3 model chemistry gave conformational enthalpy difference ΔH(e − a) in the range 0.6 and 1.8 kJ mol−1. Infrared and Raman intensities, polarization ratios and vibrational frequencies for the e and a conformers were calculated. The wavenumbers of the vibrational modes were derived in the anharmonic approximation in B3LYP/cc-pVTZ calculations. An average relative deviation of ca. 1% between the observed and calculated wavenumbers for the 48 modes of the e and a conformers was found.

Conformational Behavior of CH3OC(O)SX (X = CN and SCN) Pseudohalide Congeners. A Combined Experimental and Theoretical Study

Pure methoxycarbonylsulfenyl cyanide, CH(3)OC(O)SCN (I), and methoxycarbonylsulfenyl thiocyanate, CH(3)OC(O)SSCN (II), were prepared by reacting liquid CH(3)OC(O)SCl with either AgCN or AgSCN, respectively. Compounds I and II were characterized by (1)H NMR, CG-MS, and vibrational (FTIR and FT-Raman) 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 C(s) symmetry with a synperiplanar (syn) orientation of the carbonyl double bond (C=O) with respect to both the CH(3)O- 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 DeltaH(0) = H(0)((syn-anti)) - H(0)((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.

The Limitations of Certain Density Functionals in Modeling Neutral Water Clusters

We have used density functional theory (DFT) with four different functionals and with basis sets optimized for studying water clusters to calculate the structures and energies for selected water dimer, trimer, tetramer, pentamer, hexamer, and octamer structures. We compare the results to the CBS‐APNO and G3 model chemistry methods, and with highly accurate MP2 complete basis set limit energies. We find that while all the DFT methods capture the minimum energy structures for tetramers and pentamers, and reproduce the interaction energies well, they fail to find certain structures where London dispersion forces are critical to the interaction. Specifically structures that are not composed of cyclic rings, such as the tetramer pyramid and the pentamer cage structures, are not minima on the DFT potential energy hypersurfaces.

Computationals-Block Thermochemistry with the Correlation Consistent Composite Approach

The correlation consistent composite approach (ccCA) is a model chemistry that has been shown to accurately compute gas-phase enthalpies of formation for alkali and alkaline earth metal oxides and hydroxides (Ho, D. S.; DeYonker, N. J.; Wilson, A. K.; Cundari, T. R. J. Phys. Chem. A 2006, 110, 9767). The ccCA results contrast to more widely used model chemistries where calculated enthalpies of formation for such species can be in error by up to 90 kcal mol-1. In this study, we have applied ccCA to a more general set of 42 s-block molecules and compared the ccCA DeltaHf values to values obtained using the G3 and G3B model chemistries. Included in this training set are water complexes such as Na(H2O)n+ where n = 1 - 4, dimers and trimers of ionic compounds such as (LiCl)2 and (LiCl)3, and the largest ccCA computation to date: Be(acac)2, BeC10H14O4. Problems with the G3 model chemistries seem to be isolated to metal-oxygen bonded systems and Be-containing systems, as G3 and G3B still perform quite well with a 2.7 and 2.6 kcal mol-1 mean absolute deviation (MAD), respectively, for gas-phase enthalpies of formation. The MAD of the ccCA is only 2.2 kcal mol-1 for enthalpies of formation (DeltaHf) for all compounds studied herein. While this MAD is roughly double that found for a ccCA study of >350 main group (i.e., p-block) compounds, it is commensurate with typical experimental uncertainties for s-block complexes. Some molecules where G3/G3B and ccCA computed DeltaHf values deviate significantly from experiment, such as (LiCl)3, NaCN, and MgF, are inviting candidates for new experimental and high-level theoretical studies.

In search of CS2(H2O)n=1–4 clusters

Gaussian-3 and MP2/aug-cc-pVnZ methods have been used to calculate geometries and thermochemistry of CS(2)(H2O)n, where n=1-4. An extensive molecular dynamics search followed by optimization using these two methods located two dimers, six trimers, six tetramers, and two pentamers. The MP2/aug-cc-pVDZ structure matched best with the experimental result for the CS(2)(H2O) dimer, showing that diffuse functions are necessary to model the interactions found in this complex. For larger CS(2)(H2O)n clusters, the MP2/aug-cc-pVDZ minima are significantly different from the MP2(full)6-31G* structures, revealing that the G3 model chemistry is not suitable for investigation of sulfur containing van der Waals complexes. Based on the MP2/aug-cc-pVTZ free energies, the concentration of saturated water in the atmosphere and the average amount of CS(2) in the atmosphere, the concentrations of these clusters are predicted to be on the order of 10(5) CS(2)(H2O) clusters.cm(-3) and 10(2) CS(2)(H2O)(2) clusters.cm(-3) at 298.15 K. The MP2/aug-cc-pVDZ scaled harmonic and anharmonic frequencies of the most abundant dimer cluster at 298 K are presented, along with the MP2/aug-cc-pVDZ scaled harmonic frequencies for the CS(2)(H(2)O)(n) structures predicted to be present in a low-temperature molecular beam experiment.

Ab initio and Density Functional Study of Thionitroso XNS and Thiazyl Isomers XSN, X: H, F, Cl, Br, OH, SH, NH2, CH3, CF3, and SiF3.

Standard enthalpies of formation of several thionitroso (XNS) and thiazyl (XSN) isomers, with X = H, F, Cl, Br, OH, SH, NH 2 , CH 3 , CF 3 , and SiF 3 , were determined using coupled cluster (CC) theory with Dunning's correlation consistent basis sets cc-pVXZ and aug-cc-pV(X+d)Z, G3 and CBS-QB3 model chemistries, as well as the B3LYP DFT method. The results support the idea that the electronegativity of the bonding atom in the substituent is correlated, albeit not perfectly, with the relative stability of the XNS over the XSN isomer. detailed study was performed on the parent isomers HNS and HSN. They exhibit a singlet 1 A' ground state (as all the other molecules) at 5.4 and 9.4 kcal/mol below their first excited 3 A state, respectively (CCSD(T)/CBS calculations). The enthalpies of formation of the isomers at the CCSD(T)/CBS limit, including core valence correlation, and spin-orbit splitting, are f H° 2 9 8 (HNS) = 55.3 ′ 1 kcal/mol and Δ f H° 2 9 8 (HSN) = 75.4 ′ 1 kcal/mol. The activation energy at 0 K for the HNS → HSN isomerization was determined as 63.0 and 64.2 kcal/mol at the CCSD(T)/CBS and B3LYP/6-311+G(3df,2p) levels of theory, respectively. Additional calculations were performed for the cationic isomeric pairs XNS + /XSN + with the general result that ionization increases the relative stability of the thionitroso isomer with respect to the thiazyl isomer. In the cases of ClSN and BrSN, for which the thiazyl isomer is the most stable neutral species, the thionitroso isomer is more stable for the cations. The average deviation of the B3LYP/6-311+G(3df,2p) ionization potentials with respect to the G3 model chemistry is only 0.1 eV.

Ab Initio and Density Functional Study of Thionitroso XNS and Thiazyl Isomers XSN, X = H, F, Cl, Br, OH, SH, NH2, CH3, CF3, and SiF3

Standard enthalpies of formation of several thionitroso (XNS) and thiazyl (XSN) isomers, with X = H, F, Cl, Br, OH, SH, NH2, CH3, CF3, and SiF3, were determined using coupled cluster (CC) theory with Dunning's correlation consistent basis sets cc-pVXZ and aug-cc-pV(X+d)Z, G3 and CBS-QB3 model chemistries, as well as the B3LYP DFT method. The results support the idea that the electronegativity of the bonding atom in the substituent is correlated, albeit not perfectly, with the relative stability of the XNS over the XSN isomer. A detailed study was performed on the parent isomers HNS and HSN. They exhibit a singlet 1A‘ ground state (as all the other molecules) at 5.4 and 9.4 kcal/mol below their first excited 3A‘ ‘ state, respectively (CCSD(T)/CBS calculations). The enthalpies of formation of the isomers at the CCSD(T)/CBS limit, including core valence correlation, and spin−orbit splitting, are ΔfH°298(HNS) = 55.3 ± 1 kcal/mol and ΔfH°298(HSN) = 75.4 ± 1 kcal/mol. The activation energy at 0 K for the HNS → HSN isomerization was determined as 63.0 and 64.2 kcal/mol at the CCSD(T)/CBS and B3LYP/6-311+G(3df,2p) levels of theory, respectively. Additional calculations were performed for the cationic isomeric pairs XNS+/XSN+ with the general result that ionization increases the relative stability of the thionitroso isomer with respect to the thiazyl isomer. In the cases of ClSN and BrSN, for which the thiazyl isomer is the most stable neutral species, the thionitroso isomer is more stable for the cations. The average deviation of the B3LYP/6-311+G(3df,2p) ionization potentials with respect to the G3 model chemistry is only 0.1 eV.

Density functional study of the decomposition pathways of nitroethane and 2-nitropropaneElectronic supplementary information (ESI) available: The structure of minima on the PES of nitroethane (Fig. S1) and 2-nitropropane (Fig. S2). See http://www.rsc.org/suppdata/cp/b3/b300275f/

relatively high barriers, 42.0 kcal mol � 1 and 39.2 kcal mol � 1 for 1 and 2 respectively. These values agree well with the activation energies predicted by Benson and co-workers on the basis of experimental information. The enthalpies of formation of 1 and 2 were also determined, both applying the G3 model chemistry to the atomisation reactions on the one hand, and the G3 and B3LYP methods to isodesmic reactions involving nitromethane on the other. Supporting evidence was found that the enthalpies of formation calculated in this way for 1, � 24.5 � 1 kcal mol � 1 , and 2, � 34.0 � 1 kcal mol � 1 , are indeed accurate enough for practical purposes. The accuracy of the density functional calculations is discussed at the light of the available experimental results.

Ferromagnetism in noble metal alloys containing manganese

The infrared spectra of 1-chloro-1-silacyclohexane have been studied as a vapour and liquid at ambient temperature and as amorphous and annealed crystalline solids at 78 K. Various infrared bands present in the vapour and liquid states vanished in the crystalline state upon cooling. Raman spectra of the liquid were recorded at 293 K and polarization data obtained. Additional Raman spectra were recorded at various temperatures between 293 and 163 K, and intensity changes with temperature of certain Raman bands were detected. A crystalline phase was observed around 140 K, leading to spectral shifts and a number of vanishing bands.The compound exists in two conformers, equatorial (e) and axial (a) in the fluid phases, but only the a-conformer was present in the crystal. The experimental results suggest that the a-conformer has 2.8 kJ mol−1 lower enthalpy than e in the liquid, leading to 69% of the a conformer at 293 K.B3LYP calculations with various basis sets and the G3 model chemistry gave conformational energy difference ΔE (e − a) in the range 0.4–1.4 kJ mol−1. Infrared and Raman intensities, polarization ratios and vibrational frequencies for the e and a conformers were calculated. The fundamental wavenumbers were also derived in the anharmonic approximation in B3LYP/cc-pVTZ calculations, a relative deviation of less than 2% between the observed and calculated wave numbers for the 48 modes of the e- and a-conformers was obtained.