Showing papers in "Journal of Molecular Structure-theochem in 1982"

Gaussian basis sets which yield accurate Hartree—Fock electric moments and polarizabilities

Abstract: Calculated dipole, quadrupole and octopole moments and dipole polarizabilities are reported for H2, CH4, NH3, H2O, HF, HCN, H2CO, H3CF and HCOOH in the Hartree—Fock approximation. Nine different basis sets are employed, ranging from a minimal basis to a large triply polarized basis. It is argued that in order to achieve an accuracy of 0.001 hartree in intermolecular interaction energies, the errors in computed monomer moments should be less than 0.03, 0.15 and 0.75 au for μ, θ and Ω, respectively, while α should be correct to within 2 a03. Measured by these standards, the DZPP and larger sets provide results close to the Hartree—Fock limit, but the commonly used energy-optimized DZP basis proves unsatisfactory. An alternative moment-optimized DZP′ basis is proposed which is shown to be the smallest basis yielding results acceptably close to the Hartree—Fock limit. Vibrational corrections are estimated for the hydrides and turn out to be small. Comparison w ith experiment then shows that correlation corrections are important, especially for μ and α. Details on contraction schemes and standard scale factors for the different basis sets are given in the Appendix.

Estimates for systematic empirical corrections of consistent 4–21G ab initio geometries and their correlations to total energy group increments

Abstract: The structural parameters of the completely relaxed 4–21G ab initio geometries of more than 30 basic organic compounds are compared to experimental results. Some ranges for systematic empirical corrections, which relate 4–21G bond distances to experimental parameters, are associated with total energy increments. In general, for the currently feasible comparisons, the following corrections can be given which relate calculated distances to experimental r g parameters and calculated angles to r s -structures For CC single bond distances, deviations between calculated and observed parameters ( r g ) are in the ranges of −0.006(2) to −0.010(2) A for normal or unstrained hydrocarbons; −0.011(3) to −0.016(3) A for cyclobutane type compounds; and +0.001(5) to +0.004(4) A for CH 3 conjugated with CO. For CO single bonds the ranges are −0.006(9) to +0.002(3) A for CO conjugated with CO; and −0.019(3) to −0.027(9) A for aliphatic and ether compounds. A very large and exceptional discrepancy exists for the highly strained ethylene oxide, r s — r e = −0.049(5) A and in CH 3 OCH 3 and C 2 H 5 OCH 3 the r s — r e differences are −0.029(5), −0.040(10) and −0.025(10) A. Some of these discrepancies may also be due to deficiencies of the microwave substitution method caused by atomic coordinates close to inertial planes. For CN bonds, two types of NCH 3 corrections are from +0.005(6) to −0.006(6) and from −0.009(2) to −0.014(6) A; and the range for NCO is +0.012(3) to +0.028(4) A. For isolated CC double bonds the range is + 0.025(2) to +0.028(2) A. For conjugated CC double bonds the correction is less positive (+0.014(1) A for benzene). For CO double bonds the corrections are −0.004(3) to +0.003(3) A. For bond angles of type HCH, CCH, CCC, CCO, CCO, OCO, NCO and CCC the corrections are of the order of magnitude about 1–2° (or better). Angles centered at heteroatoms are less accurate than that, when hydrogen atoms are involved. Differences in HOC and NHC angles were found in a range of −2.3(5)° to −6.2(4)°.

Use and abuse of the distinguished-coordinate method for transition-state structure searching

Abstract: Exploration of potential-energy surfaces for reacting systems is often expedited by selection of a distinguished coordinate, to measure progress between reactants and products, at each value of which the remaining coordinates are optimized by constrained energy-minimization. A classification of surfaces is suggested according to the number of energy minima in the subspace excluding the distinguished coordinate in the saddle region. Type S surfaces are well behaved in the distinguished-coordinate method but type D surfaces are not. In general, reaction paths and energy profiles generated by this method for type D surfaces are discontinuous and cannot be used to locate transition-state structures precisely. An extension of the method using two distinguished coordinates is discussed in relation to the example of formaldehyde hydration but is found to suffer from the same pathological condition in two dimensions as does the standard method in one dimension.

The structure of some nitrogen heteroaromatics

Abstract: The structures of pyridine, pyrimidine, pyrazine, and s-triazine have been determined by the ab initio gradient method with a 4–21 basis set augmented by addition of polarization functions on the nitrogen atoms. The effect of omission of the polarization functions is also investigated. Corrections have been applied to yield computed r 0 structures, assuming approximate constancy of the effects of neglect of electron correlation, use of a finite basis set, and correction for zero-point vibrations. The validity of these assumptions is carefully evaluated, and the results are shown to agree with the best available experimental evidence to a high degree of accuracy. The effect of nitrogen atoms on the ring structure is quite systematic, with a sharp decrease in the ring angles at nitrogen, becoming progressively greater as more nitrogen atoms are substituted for carbon. There is little change in CN distances between the compounds.

Le calcul de l'energie de stabilisation des especes chimiques

Abstract: In the course of our theoretical studies on chemical intermediates we have been led to re-examine the concept of stability commonly used by experimentalists, often without explicit definition. In this work we generalize the thermochemical definition of the conventional stabilization energy SE 0 = Δ H 0 a − {dcΣ| l }ϵ 0 l Using experimental or semi-empirical heats of formation we are able to calculate the stabilization energy of any kind of chemical species - saturated and conjugated molecules, free radicals, carbocations and carbanions - and to analyse the influence of substituents on the properties of these systems. Our definition is compared with others less general but more commonly used.

Theoretical studies of the structures of some mono- and di-substituted benzenes

Abstract: The structures of some monosubstituted benzenes have been optimized at the ab initio STO—3G level, and, in some cases, also using split valence base sets. It is found that the geometries obtained are in reasonable agreement with experimental gas phase values, particularly as far as substituent induced trends are concerned. The work has been extended to para-disubstituted benzenes to examine the contribution of quinonoid forms. The geometrical evidence and interaction energies obtained indicate that such forms are of minor significance in the neutral molecules, but can be quite significant in ions such as the para-nitrophenoxide anion.

STO-3G MO calculations on structures and internal rotational barriers of phenol, benzoyl X(X = H, F, CH3, CN, OCH3), acetyl fluoride, acetyl cyanide, and carbonyl cyanide

Abstract: The side chain geometry and some adjacent bond lengths and angles of the ring are optimized at the STO-3G level of molecular orbital theory for the planar and orthogonal forms of benzoyl X(X = H, F, CN, CH3, OCH3). Similar calculations are reported for acetyl fluoride, acetyl cyanide, and carbonyl cyanide, for which experimental structures and reliable internal barriers are available. The calculated barriers for the benzoyl compounds suggest steric hindrance by X in the ground state as a major cause of the variation in the barrier magnitudes. Good agreement between calculated and experimental geometries for acetyl cyanide and carbonyl cyanide, as well as for the internal rotational barrier in the former, are taken to imply a reliable calculated geometry for benzoyl cyanide. A total geometry optimization for phenol agrees fairly well as for the internal rotational barrier in the ture and also with the direction and magnitude of the dipole moment. Optimization of the ring geometry does not lower the calculated internal rotation barrier.

A theoretical investigation of the sulfur—selenium bond

Abstract: A theoretical ab initio study of HSH, HSeH, HOOH, HOSH, HSSH, HSSeH, HSeSeH, HSSSH, HSeSSH, HSSeSH, HSeSeSH, HSeSSeH and HSeSeSeH has been performed using minimal Gaussian basis sets. Full geometry optimization has been carried out for HSH and HSeH. For the four-atom molecules only the chalcogen —chalcogen bond length and the dihedral angle have been optimized while other bond parameters are held fixed at experimental values. The geometries of the five-atom molecules have not been optimized. The calculated bond parameters of HSH and HSeH are in good agreement with the experimental values. The optimum XY (X, Y  S, Se) bond lengths are somewhat larger than the values estimated from IR and microwave data. The electronic structures of the molecules indicate that the SS, SeS and SeSe bonds are very similar. The total energy change calculated for the interconversion reaction between these bonds is small, as indicated by experimental studies. This may well explain why the rearrangement reactions between various Se n S 8— n molecules are possible both in solution and in the molten state. A possible mechanism for the rearrangement is discussed.

Geometry of molecules: Part 5. Interatomic distances and electronic structures of some aloukyl-substituted cyclopropanes and cyclopropenes by the IMOA method

Abstract: The geometric and electronic structures of some alkyl- and vinyl-substituted cyclopropanes and cyclopropenes have been computed by the iterative maximum overlap approach (IMOA). Some MINDO/3 calculations have been carried out for comparison. Asymmetries introduced in the three-membered rings by the substitution are discussed in detail. Comparison of the predicted bond distances in vinylcyclopropane with microwave data indicates the presence of a conjugative HOMO-LUMO interaction between the strained ring and the ethylene moiety. It is stressed that interatomic distances and bond lengths should be distinguished in strained systems. The bond lengths are defined as segments of curves passing through the points of maximum electron density and the relevant nuclei. They are calculated and discussed for some characteristic highly strained rings. The estimated heats of formation and strain energies are in satisfactory agreement with available experimental data.

Ab initio studies of structural features not easily amenable to experiment: Part 20. Structural aspects of ethyl groups

Abstract: The molecular structures of a number of stable conformations of ethanol, ethylamine, methylethyl ether, methylethylamine and of the ethyl anion have been determined by ab initio geometry optimizations using Pulay's Force method on the 4–21G level. The calculated geometries characterize the extent to which structural groups in a molecule are sensitive to asymmetries in their environment. Characteristic structural trends are consistently found for the CH bond distances and CCH angles in the C2H5 groups of trans-ethanol, trans-methylethyl ether and in the ethyl anion. They differ from those previously found for C2H5 groups in hydrocarbons. There is qualitative disagreement between the trends calculated for CH bond distances in trans-ethanol and trans-methylethyl ether and those found in the microwave substitution structures of these compounds. Since the substitution parameters are unresolved because of relatively large experimental or model uncertainties, it is presently impossible to decide whether this discrepancy is the result of computational or experimental deficiency. The methyl groups in methylethyl ether and methylethylamine exhibit the characteristic structural distortions which are usually found for CH3 groups adjacent to electron lone pairs. The CC bond distances in C2H5 in the systems studied here are sensitive to the conformational arrangement of ethyl relative to the rest of a system in a way which can be rationalized by orbital interactions involving antibonding orbitals on sp3-hybridized carbon atoms. The calculated conformational stabilities agree qualitatively with experimental trends, except in the case of ethanol where the trans — gauche energy difference is small (about 0.1 kcal mol−1) and within the uncertainties of the calculations. Our conformational energies for CH3CH2NH2 are in disagreement with a previous ab initio investigation based on a comparison of unoptimized standard geometries. In general, the agreement between calculated structural parameters and corresponding reliable experimental values is very good in all comparable cases.

Force field and vibrational assignment for cyclobutane from a combination of ab initio calculations and experimental data

Abstract: The geometry, force constants and dipole moment derivatives for cyclobutane have been calculated ab initio at the Hartree—Fock level using a 4–21 Gaussian basis set and the gradient method. To account for systematic deviations, the calculated force constants are scaled first and the assignment of the fundamental frequencies of cyclobutane-d 0 and -d 8 is analysed. Besides some minor reassignments, the A 1 rocking fundamentals are assigned to significantly higher frequencies than previously. The final refined force field is determined by optimizing the scale factors on the experimental frequencies.

Structure and conformations of 1,4-Cycloheptadiene, 1,3-cycloheptadiene and 1,3,5-cycloheptatriene

Abstract: The fully optimized structures of the molecules 1,4-cycloheptadiene, 1,3-cycloheptadiene and 1,3,5-cycloheptatriene have been determined by ab initio gradient calculations at the double zeta level. The most stable conformation of 1,4-cycloheptadiene has C2 symmetry, but a second form with Cs symmetry exists at approximately 10 kcal mol−1 higher energy. The most stable form of 1,3-cycloheptadiene has Cs symmetry with all but one carbon atom approximately in the same plane. This form was found to be 2.5 kcal mol−1 more stable than a form with C2 symmetry. A third stable conformer, also of C2 symmetry, exists at considerably higher energy, 1,3,5-Cycloheptatriene exists in a boat form with Cs symmetry with a barrier to inversion through the planar form of 5.7 kcal mol−1. The most important factors determining the ring conformations are angle strain and non-bonded interactions, with conjugation effects relatively small. For molecules with adjacent methylene groups, the most important factor seems to be adjustment of the torsional angles to minimize the interaction between these groups.

A theoretical study of molecular structures and electronic properties of aldoximes: CH2NOH, CH3CHNOH, NH2CHNOH, OHCHNOH and FCHNOH

Abstract: Results of ab initio SCF calculations using a double-zeta (DZ) basis set are reported for aliphatic aldoximes RCHNOH (R  H, CH3, NH2, OH and F). The most stable conformer and the energy difference between the anti and syn conformers have been determined by fully optimizing geometrical parameters. The effect of substituents on the electronic charge distribution of these molecules is studied in terms of calculated electronic properties.

Quantitative frontier orbital theory: Part I. Electrophilic aromatic substitution

Abstract: An ab initio version of frontier orbital theory which employs the charges within defined regions of space associated with particular orbitals is described. It is then applied to nitration of a wide range of aromatic molecules.

Non-empirical SCf molecular orbital studies on simple zeolite model systems

Abstract: Non-empirical molecular orbital studies on the relative stabilities of topologically closed ring clusters H 8 Si 4 O 4 , [H 8 Si 3 AlO 4 ] − and [H 8 Si 2 Al 2 O 4 ] − modelling building units of zeolite frameworks, have been carried out. According to the calculations, AlOAl type bridges are unstable in doubly negative charged species, but can be stabilized in the presence of cations. This stabilization effect increases with increasing cation charge. Several hypothetical bimolecular cluster equilibria are also discussed.

The structure of vinylamine

Abstract: Theoretical structures for vinylamine have been obtained by ab initio molecular orbital calculations, using the minimal STO—3G, split-valence 3—21G, and split-valence plus d-polarization 6—31G* basis sets. The theoretical structures were corrected for systematic deficiencies in the basis sets and by use of experimental rotational constants to predict a complete r0 structure for vinylamine. Vinylamine is predicted to be non-planar with a pyramidal amino group.

Ab initio study of rotational isomerism in vinylcyclopropane

Abstract: The geometry of vinylcyclopropane has been completely optimized at each critical point by analytic gradient (force) methods at the minimal STO-3G and the split-valence 3-21G basis set levels. The geometries obtained for the various critical points have been used to generate potential energy curves for vinyl group rotation within the rigid rotor approximation. Comparison of these curves clearly demonstrates the importance of complete geometry optimization. The potential energy curve for vinyl group rotation, generated with the s-trans STO-3G optimized geometry, predicts secondary gauche minima which are an artifact of the rigid rotor approximation. With complete geometry optimization along the curve, the STO- 3G basis set computations predict only s-trans and s-cis minima. In contrast, the complete optimizations with the 3-21G basis set, in agreement with experiment, predict a three-fold rotational contour with two equivalent gauche minima. These minima lie 6.86 kJ mol−1 above th e s-trans minimum. The computed barrier to rotation for the s-trans → gauche interconversion is 13.3 kJ mol−1. The electric dipole moment computed with the 3-21G basis for the s-trans 3-21G optimized geometry is 0.446 D or about 10% less than the experimental value.

Molecular fragmentations: Part VIII. Structures and energies of mass spectral fragments derived from tetraphosphorus trisulphide☆

Abstract: Molecular structures and energies have been calculated in the MNDO approximation, for P4S3 and its molecular ion P4S3+, and for the mass spectral fragment pairs: (P3S3+ + P), (P3S2+ + PS), (P3S+ + PS2), (P2S3+ + P2), (P2S2+ + P2S), (P2S+ + P2S2), (P2S2), PS3+ + P3), (PS2+ + P3S), (PS+ + P3S2), and (PS+ + P2S + PS). Three distinct energy minima were found for each of P2S2+ and P2S2, and two minima for each of P2S+, P2S, PS3+, PS3+, PS2+, PS2, P3+ and P3. The fragments arising from P4 and P4+ were also investigated. The structures are discussed in terms of the Jahn—Teller effect, whose predictions are fulfilled without exception.

The molecular structure, vibrational force field, spectral frequencies, and infrared intensities of CH3POF2

Abstract: The molecular structure of CH3POF2 has been determined by MOLCAOSCF single determinant calculations using split valence shell basis sets with polarization functions on phosphorus. Interpretation of the results was aided by similar structural calculations on CH3POH2, CH3PF2, CH3PH2 and POF3. In all cases, geometry optimization was performed by the gradient method using the TEXAS program. Comparison with experimental microwave results for CH3POF2 reveals an apparent error of interpretation in the experimental study leading to a radically incorrect angular distribution of bonds around the phosphorus atom. The vibrational force field of CH3POF2 has been determined by numerical differentiation of analytically determined energy gradients and used to compute the molecular vibrational spectrum and to provide approximate mode descriptions of the vibrational transitions. Agreement between the calculated and observed frequencies is reasonable and can be made very satisfactory by evaluation of a small number of scale factors to remove the systematic error common to vibrational frequencies calculated at this level of approximation. Dipole moment derivatives are also calculated and used to predict IR intensifies.

Determination of force fields for formaldehyde, acetaldehyde and acetone by the CNDO/force method

Abstract: CNDO/Force calculations have been done for formaldehyde, acetaldehyde and acetone, and the theoretical force fields evaluated. Experimental force fields are obtained from vibrational frequencies using the least-squares refinement method. The initial force fields considered are based on the bending and interaction force constants obtained from the CNDO/Force calculations and the stretching force constants transferred from chemically related molecules. Vibrational frequencies of H 2 CO, D 2 CO, HDCO, H 2 13 CO and D 2 13 CO for formaldehyde, CH 3 CHO, CH 3 CDO, CD 3 CHO, CD 3 CDO and CH 2 DCHO for acetaldehyde, and CH 3 COCH 3 CD 3 COCH 3 and CD 3 COCD 3 for acetone are employed in the force field refinements. The final force fields obtained are found to be reasonable with respect to the diagonal and interaction force constants.

An ab initio SCF and CI study of ketene imine

Abstract: The electronic structures of the ground and excited states of ketene imine (HHCCNH) have been studied by ab initio SCF and CI calculations. The nucleophilic nature of the β carbon with respect to nitrogen has been discussed using calculated electrostatic potentials and by calculated energy differences between the parent and protonated species. The electronically excited 1A″ and 3A″ states are found to be almost degenerate.

The boron analogue of glycine: A theoretical investigation of structure and properties

Abstract: Ab initio calculations have been performed on the isolated molecule and dimer of NH 3 · BH 2 · CO 2 H. Optimum geometries, dipole moments, orbital energies, electronic configuration, charge distribution and electrostatic molecular potentials are reported and the results compared with those of similar calculations on glycine and protonated glycine. The results of conformational studies using PCILO are also reported.

The electronic structure of nickel carbide

Abstract: A sester (2.5) zeta basis set, which is double zeta in the inner shells and triple zeta in the valence shell, has been used in the ab initio Generalized Valence Bond (GVB) calculations on NiC. The molecule has a closed electronic shell with a formal triple bond and two lone pairs (:NiC:). The following structural parameters have been obtained: r0 = 1.805 A, E0 = −1544.0934 hartrees, De = 23 kcal mol−1 and ωe = 1219 cm−1.

Ligands of low electronegativity in the vsepr model: molecular pseudohalides

Abstract: Equilibrium structures and force constants at linearity, for the skeletal bending mode δ (RNX) have been calculated in the MNDO approximation for 67 isocyanates, isothiocyanates and azides, RNXY (XY = CO, CS or N 2 ) and the corresponding structures and force constants, δ (RCN), for 12 fulminates RCNO. Fulminates all have linear skeletons, but for RNXY the molecular skeleton is linear at atom X only if it is linear at N also; otherwise the skeleton RNXY has a trans planar structure. Bending force constants are large and negative for all azides studied, negative for methyl and substituted methyl isocyanates and isothiocyanates and very small and positive for silyl and substituted silyl isothiocyanates: for silyl and substituted silyl isocyanates, the force constant is small and positive when the R group has effective C 3v symmetry, but small and negative when the R group has only effective C s symmetry.

The quantum chemical calculation of environmental effects: A comparative study of charge separation in water dimers

Abstract: The numerical results of two forms of the inhomogeneous self-consistent reaction field theory of surrounding medium effects are compared. Both theories are based on the CNDO/INDO approximate MO scheme and are tested on prototypes for spatial charge separation devices, namely water dimers. The CNDO/2 data are compared with those of ab initio calculations performed on the same systems under identical conditions. Ab initio and semiempirical results are discussed in terms of the well known two-state model for H bonding. In general, H bonding between the components of an ion pair leads to a weakening of the surrounding medium effects, while the opposite is found for uncharged partners in canonical H bonds.

The molecular and electronic structure of bicyclo-[1,1,0]butane, bicyclo[2,1,0]pentane, and bicyclo[3,1,0]-hexane as obtained by ab initio SCF calculations

Abstract: Ab initio SCF calculations with force relaxation using a 4-31G basis has been applied in a complete geometry optimization of the molecules bicyclo[1,1,0]butane (BCB), bicyclo[2,1,O]pentane (BCP), and bicyclo[3,1,0]hexane (BCH). Only the boat form of the last molecule was considered. For BCB complete optimizations using 6-31G and 6-31G* basis sets were also carried through. For all basis sets used the difference between the two inequivalent CC bond distances was predicted to be ∼ 0.02 A, the transannular bond being the shorter, whereas experimental results [1] suggest two equal bond lengths. In the case of BCP our results unequivocally support the microwave structure [3] rather than the electron diffraction model [2]. Our optimized structure for BCH is in rather good agreement with results from recent experimental studies [7] the only serious discrepancy being the transannular bond where our value of 1.509 A is significantly longer than the experimental value of 1.454 A. For all the molecules studied we have optimized all independent structure parameters individually. The results have for the first time provided information on the local symmetry of the CH2-groups in these ring compounds.

Electron densities in simple organic molecules

Abstract: Dipole moments and atomic electron populations have been calculated for a series of simple organic molecules by ab initio molecular orbital theory using various basis sets. The effect of geometry optimization on these properties is examined. It is shown that values obtained from STO-3G and, particularly, 4-31G basis sets using standard geometries correlate well with those from molecules optimized at the 6-31G* level. Atomic electron populations derived by the normal Mulliken analysis are shown to follow those obtained by topological procedures within restricted series.