Representations of molecular force fields. I. Ethane: Ab initio and model, harmonic and anharmonic

The quadratic and selected cubic force constants for ethane have been computed, using single determinant molecular orbital wavefunctions at the 4‐31G level, with a view to testing and extending model consistent force fields (CFF) for ’’molecular mechanics’’ calculations. Results agree semiquantitatively with experiment, but experimental force constants of sufficient reliability to provide a definitive comparison are not yet available. In a comparison with the most rational general CFF available, that of Ermer and Lifson, the most significant discrepancies found to occur are those for certain stretch–bend couplings assumed to be zero in the CFF but shown to be appreciable by quantum calculation. It is observed that these couplings, but not the stretch–stretch couplings, are well accounted for by a steric interaction model. The ab initio cubic constants examined display the same pattern of conformity with a steric model. Bend–bend–bend and bend–bend–stretch but not all stretch–stretch–stretch interactions agree with those of the steric model. The partial success of the steric model shows that it is possible to represent a large number of interaction constants, quadratic and higher order, by a small number of parameters in molecular mechanics. The failure of the steric model to account for predominantly stretching interactions confirms that ’’classical’’ nonbonded interactions as embodied in conventional Urey–Bradley fields are not the only major contributors to off‐diagonal force constants. An alternative model, the anharmonic model of Warshel, as modified by Kirtman et al., was found to account well for pure stretches but not for bends or stretch–bend interactions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70643/2/JCPSA6-63-11-4750-1.pd

Self‐consistent molecular orbital methods. XXIII. A polarization‐type basis set for second‐row elements

The 6‐31G* and 6‐31G** basis sets previously introduced for first‐row atoms have been extended through the second‐row of the periodic table. Equilibrium geometries for one‐heavy‐atom hydrides calculated for the two‐basis sets and using Hartree–Fock wave functions are in good agreement both with each other and with the experimental data. HF/6‐31G* structures, obtained for two‐heavy‐atom hydrides and for a variety of hypervalent second‐row molecules, are also in excellent accord with experimental equilibrium geometries. No large deviations between calculated and experimental single bond lengths have been noted, in contrast to previous work on analogous first‐row compounds, where limiting Hartree–Fock distances were in error by up to a tenth of an angstrom. Equilibrium geometries calculated at the HF/6‐31G level are consistently in better agreement with the experimental data than are those previously obtained using the simple split‐valance 3‐21G basis set for both normal‐ and hypervalent compounds. Normal‐mode vibrational frequencies derived from 6‐31G* level calculations are consistently larger than the corresponding experimental values, typically by 10%–15%; they are of much more uniform quality than those obtained from the 3‐21G basis set. Hydrogenation energies calculated for normal‐ and hypervalent compounds are in moderate accord with experimental data, although in some instances large errors appear. Calculated energies relating to the stabilities of single and multiple bonds are in much better accord with the experimental energy differences

Self‐consistent molecular orbital methods. XXIII. A polarization‐type basis set for second‐row elements

The 6‐31G* and 6‐31G** basis sets previously introduced for first‐row atoms have been extended through the second‐row of the periodic table. Equilibrium geometries for one‐heavy‐atom hydrides calculated for the two‐basis sets and using Hartree–Fock wave functions are in good agreement both with each other and with the experimental data. HF/6‐31G* structures, obtained for two‐heavy‐atom hydrides and for a variety of hypervalent second‐row molecules, are also in excellent accord with experimental equilibrium geometries. No large deviations between calculated and experimental single bond lengths have been noted, in contrast to previous work on analogous first‐row compounds, where limiting Hartree–Fock distances were in error by up to a tenth of an angstrom. Equilibrium geometries calculated at the HF/6‐31G level are consistently in better agreement with the experimental data than are those previously obtained using the simple split‐valance 3‐21G basis set for both normal‐ and hypervalent compounds. Normal‐mode vibrational frequencies derived from 6‐31G* level calculations are consistently larger than the corresponding experimental values, typically by 10%–15%; they are of much more uniform quality than those obtained from the 3‐21G basis set. Hydrogenation energies calculated for normal‐ and hypervalent compounds are in moderate accord with experimental data, although in some instances large errors appear. Calculated energies relating to the stabilities of single and multiple bonds are in much better accord with the experimental energy differences

Software for the frontiers of quantum chemistry:An overview of developments in the Q-Chem 5 package

This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange–correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear–electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an “open teamware” model and an increasingly modular design

An Ab Initio Theory and Density Functional Theory (DFT) Study of Conformers of Tetrahydro-2H-Pyran

Ab initio theory with the 3-21G, 6-31G(d), 6-31G(d,p), 6-311G(d,p), 6-31+G(d), and 6-311+G(d,p) basis sets and density functional theory (SVWN, pBP, BLYP), including the hybrid density functional methods B3LYP, B3PW91, and B3P86, have been used to calculate the energies of the chair, half-chair, sofa, twist, and boat conformers of tetrahydro-2H-pyran (oxacyclohexane, oxane, pentamethylene oxide, tetrahydropyran, THP). The enthalpies (ΔH°), entropies (ΔS°), and free energies (ΔG°) of the conformers were also determined. The energy difference (°E, kcal/mol) between the chair and the 2,5-twist conformer is 5.92 to 6.10 (HF), 5.78 to 6.10 (MP2), 6.71 to 6.82 (SVWN), 6.04 to 6.12 (pBP), and 5.84 to 5.95 (BLYP, B3LYP, B3PW91, B3P86). The energy difference (°E, kcal/mol) between the chair and the 1,4-boat conformer is 6.72 to 7.05 (HF), 6.76 to 7.16 (MP2), 6.97 to 7.20 (SVWN), 6.26 to 6.36 (pBP), and 6.23 to 6.46 (BLYP, B3LYP, B3PW91, B3P86). The transition state between the chair conformation and the 2,5-twist conformation is 11 kcal/mol higher in energy than the chair conformer

An Ab Initio Molecular Orbital Theory Study of the Conformational Free Energies of 2-Methyl-, 3-Methyl-, and 4-Methyltetrahydro-2H-Pyran

Ab initio molecular orbital theory with the 6-31G(d), 6-31+G(d), 6-31G(d,p), 6-311G(d,p), 6-311+G(d,p), 6-31G(2d), 6-311G(2d), and 6-311G(2d,p) basis sets have been used to calculate the conformational enthalpies (ΔH°), entropies (ΔS°), and free energies (ΔG°) of the axial and equatorial conformers of 2-methyl-, 3-methyl-, and 4-methyltetrahydro-2H-pyran (tetrahydropyran, oxacyclohexane, oxane) and methylcyclohexane (toluene). Although HF and MP2 generally gave higher conformational free energies (ΔG°) than the experimentally reported values, other MP2 calculations gave ΔG° values in excellent agreement with experimental results for methylcyclohexane [6-311G(d 311G(d,p)] and 3-methyltetrahydro-2H-pyran [6-31+G(d), 6-311+G(d,p)]. Consistent with solution studies, the MP2 calculations gave larger ΔG° values for 4-methyltetrahydro-2H-pyran than for methylcyclohexane

Ab initio study of secondary isotope effects on molecular structure

An ab initio calculation of a secondary isotope effect on a bond length has been carried out for the first time. Single determinant molecular orbital computations indicate that the mean C–C bond length at 0 °K is longer in C2H6 than in C2D6 by 0.0015 Å. A comparison with model calculations supports a steric interpretation in which H atoms act as if they are bulkier than D atoms owing to their larger bending amplitudes of vibration.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69391/2/JCPSA6-63-7-3042-1.pd

An Ab Initio Molecular Orbital Study of the Conformational Energies of 2-Alkyltetrahydro-2H-Pyrans (Tetrahydropyrans, Oxacyclohexanes, Oxanes)

Ab initio Hartree-Fock and Density Functional Theory calculations were used to obtain the geometries and relative energies of the rotamers in the chair conformations of 2-alkyltetrahydro-2H-pyrans and 2- (trimethylsilyl)tetrahydro-2H-pyran. The MP2/6-31G*//6-31G* conformational energies (-ΔG°or A values, kcal/mol) of the 2-alkyltetrahydro-2H-pyrans (Me = 3.18; Et = 3.04; i-Pr = 3.03; t-Bu = 7.56; neo Pent = 2.84) and 2- (trimethylsilyl)tetrahydro-2H-pyran (SiMe3 = 4.77) are larger than those calculated for the corresponding alkylcyclohexanes and 2-alkyltetrahydro-2H- thiopyrans (tetrahydrothiopyrans, thiacyclohexanes, thianes). Plots of the calculated conformational energies for the 2-substituted tetrahydro-2H-pyrans versus the calculated -ΔG°values for the corresponding alkylcyclohexanes (slope = 1.34 and r = 0.983) and for the corresponding 2-substituted tetrahydro-2H-thiopyrans (slope = 2.01 and r = 0.986) are linear. (C) 2000 Elsevier Science B.V

Revision of the structure of acremine P from a marine-derived strain of Acremonium persicinum

The previously published structure of the fungal metabolite acremine P is revised by re-evaluation of chemical shift values and NOESY data, and by DFT calculations