Showing papers in "Journal of Molecular Structure-theochem in 1986"

Gaussian basis sets and the nuclear cusp problem

Abstract: Variational calculations of the H-atom are performed with different types of Gaussian basis sets, (a) Huzinaga basis sets of 1s-Gaussians, (b) ns basis sets with n = 1, 2, 3, 4 ⋯ and optimized common exponents for each set, (c) ns basis sets with n = 1, 3, 5 ⋯ (d) ns basis sets with n = 1, 2, 3, 5, 7 ⋯ (e) ns basis sets with n = 1, 2, 3, 4 ⋯ but with all orbital exponents optimized. The number of basis functions in a set is N . For the sets (a) the errors of the energy (upper or lower bound) and of the expectation values r k , k = −2, −1, ⋯ 5 appear to converge as ≈ exp (— b √ N ), for sets (b) and (e) as ≈exp (— bN ), for set (c) the error of the upper bound converges as ≈N −2 and for set (d) as ≈ N −4 . For medium accuracy the Huzinaga sets are surprisingly good. They are poorly suited only for representing the cusp at the nucleus and the variance of the energy. For high accuracy basis sets (b) and (e) are superior. They represent the cusp correctly. For them the energy variance converges rapidly to zero, and all other quantities converge finally much faster. Basis sets with only even powers of r (or with just 2 s included) and a common (optimized) orbital exponent are, although formally “complete in the energy space”, of no practical use. Basis sets (e) yield by far the fastest convergence. The results are analyzed and their consequences are discussed.

A New dimension to quantum chemistry: Theoretical methods for the analytic evaluation of first, second, and third derivatives of the molecular electronic energy with respect to nuclear coordinates

Abstract: Perhaps even more important to quantum chemistry than enhancements in computer technology has been the theoretical development of analytical derivative methods. It may be realistically stated that such analytic derivative methods provide a new dimension to molecular electronic structure theory. The combination of the two effects (computational advances and theoretical progress) has led to explosive growth in the field. This account primarily describes theoretical developments at Berkeley over the past five years. Analytic first, second and third derivative methods have been developed for single-configuration self-consistent-field (SCF) wavefunctions. Analytic first and second derivative techniques now exist for multiconfiguration (MC) SCF and configuration interaction (CI) wavefunctions. These methods have been used to address a variety of chemical problems, some of which are described here.

Ab initio studies of structural features not easily amenable to experiment: Part 49. Conformational analysis and molecular structures of ethylenediamine and aminoethanol

Abstract: The structures of eleven conformations of aminoethanol and of ten conformations of ethylenediamine were determined by ab initio gradient geometry optimization on the 4–21G level. The calculations show that many energetically different conformers exist for the gauche and trans forms (NCCO or NCCN torsions) of both systems. The results indicate that structural effects on NH bond distances associated with aliphatic N⋯N or NH⋯O type hydrogen bonding are less noticeable than those obtained previously for other XH⋯Y interactions. In the same way as CH and CC bonds in aliphatic compounds studied previously, NH bonds in the same NH2-group anti-periplanar to CH are found here to be slightly longer (∼0.001 A) than NH bonds antiperiplanar to CC. The most stable 4–21G conformations of ethylenediamine (NCCN ≅ 58° and 62°) are identical with the main conformer identified by others in the gas electron diffraction data of this compound (NCCN = 64° ± 4), but the calculations afford a more detailed description of the NH2 arrangements (NC torsions) than that obtained by the gas phase experimental data. Structural effects of electron lone-pair orientation are discussed. A small but potentially significant discrepancy exists between the electron diffraction rg average CC bond distance (1.545 A ± 0.008) and the calculated rg value (1.528 A ± 0.003; 4–21G average value empirically corrected as described previously). Experimental and calculated average rg CN bond distances are 1.469 A ± 0.004 and 1.463 A ± 0.006, respectively. Some aspects of the CC bond discrepancy are discussed in detail.

The hydrogen bonding influence on polarizability and hyperpolarizability. A derivative hartree-fock study of the electrical properties of hydrogen fluoride and the hydrogen fluoride dimer

Abstract: Derivative Hartree—Fock calculations have been carried out to explore the dependence of the molecular dipole polarizability and hyperpolarizability tensors of the HF dimer as a function of the intermolecular separation. The results show a diminishing axial hyperpolarizability (β) in the course of hydrogen bond formation. The polarizability (α) changes little. The changes in the hyperpolarizability can be well explained as arising from the hyperpolarizability induced by the field of one monomer via the second and third hyperpolarizability of the other. This is consistent with the important role of simple electrical interactions in hydrogen bonding and indicates that the electrical properties of the dimer can be well predicted from the electrical properties of the isolated monomers.

An ab initio calculation of the infrared spectrum and tautomerism of guanine

Abstract: An ab initio molecular orbital calculation has been made for guanine to predict its equilibrium geometry, harmonic force constants, atomic polar tensor elements and vibrational frequencies and intensities. The predicted infrared spectrum, with the 3-21G basis set and further scaled, agrees well with the recent experimental study of guanine in an argon matrix for the in-plane modes. Further calculations indicate that the complicated experimental spectrum is probably due to the existence of tautomers other than the 9-H tautomer for which the spectrum was calculated.

On the possibility of fluorescence from twisted intramolecular charge transfer states of 2-dimethylamino-6-acylnaphthalenes. A quantum-chemical study☆

Abstract: MNDO geometry, CNDO/S CI spectra and dipole moments of 2-dimethylamino-6-propionylnaphthalene (PRODAN), a representative of a class of highly environment-sensitive fluorescent probes, were computed. Only the conformer with a twisted dimethylamino group has an excited state dipole moment high enough (27 D) to explain spectral properties of PRODAN. On the basis of the Amos—Burrows theory, fluorescence from the TICT state is predicted in polar solvents.

The contribution of dispersion to H-bonds between hydrides of first and second-row atoms

Abstract: H-bonds involving NH3, PH3, OH2, and SH2 as proton acceptor and HF and HCl as donor, as well as the water dimer, are studied by ab initio molecular orbital methods. The influence of dispersion on the properties of these complexes is calculated by second-order Moller —Plesset perturbation theory using a doubly-polarized double-ξ basis set. This theoretical approach yields H-bond lengths in much better agreement with experiment than distances calculated at the SCF level. The contractions in the bond lengths introduced by dispersion grow rapidly as first-row atoms are replaced with second-row analogs. A similar trend is noted in the H-bond energies where the contribution of dispersion ranges between 22% for the smaller systems and 69% for complexes such as H2SHCl. In addition, dispersion is seen to be responsible for a large fraction of the stretch in the HX bond resulting from H-bond formation. On the other hand, the angular characteristics of these complexes are relatively unaffected by dispersion but are controlled, instead, primarily by electrostatic factors.

A periodic table for polycyclic aromatic hydrocarbons: Part IX. Isomer enumeration and properties of radical strictly peri-condensed polycyclic aromatic hydrocarbons

Abstract: Isomer enumeration of strictly peri-condensed benzenoid (PAH6) radicals is simplified by enumerating their excised internal structures. The number of many radical benzenoid isomers are enumerated for the first time. Structural properties of this important class of high-spin hydrocarbons has been detailed. It is shown that for a given number of formula carbon atoms, the strictly peri-condensed benzenoid hydrocarbon structures are the most stable, and the possible pyrolytic formation of strictly peri-condensed benzenoid hydrocarbons via condensation of other benzenoid hydrocarbons can only occur with the evolution of hydrogen. If the excised internal structure of a strictly peri-condensed PAH6 is not a cross conjugated diradical species and it has a HMO eigenvalue of zero, then its corresponding strictly peri-condensed PAH6 structure will also have an eigenvalue of zero.

The superposition error problem: the (HF)2 and (H2O)2 complexes at the SCF and MP2 levels

Abstract: The title calculations were performed with the aim of providing data for the critical examination of the utility of the basis set superposition error (BSSE) correction. The main results obtained are as follows. The SCF interaction energies corrected for the BSSE and evaluated with the basis sets of split-valence and DZ origin are similar. With the stabilization energy, both the basis set superposition error and the intersystem correlation energy are important. MP2 stabilization energies are only slightly dependent on basis set for basis sets of DZ + P or better quality. The basis set superposition error at both the SCF and MP2 levels remains almost unchanged when passing from the 6–31G* to the 6–311G(2d, 2p) basis set. At the SCF level sufficiently accurate geometries were obtained with the standard 6–31G* basis set. Optimization at the MP2 level with this and larger basis sets brings about only small changes with respect to optimum SCF geometries.

Isomers of Si2C2: an MBPT study

Abstract: The structure of the low-lying electronic states of the unknown Si 2 C 2 system is studied using full fourth-order MBPT. It is concluded that the lowest-lying structure is a rhombus, 1 A g state, which is ca. 12 kcal mol −1 lower than the linear 3 Σ g − state. This is similar to the C 4 system. The rhombus structure accommodates reluctance of silicon to form stable multiple bonds. Vibrational frequencies are reported.

Ab initio calculations of the rotational barriers in formamide and acetamide: The effects of polarization functions and correlation

Abstract: Ab initio calculations have been used to determine the gas-phase rotational barrier about the CN bond in formamide and acetamide. The results indicate that the inclusion of polarization functions in the basis set leads to a substantial decrease (ca. 5 kcal mol −1 ) in the calculated barrier height at the SCF level. Electron correlation effects decrease the barrier by less than 1 kcal mol −1 , while the addition of zero point energy corrections changes the barrier height only slightly. Based upon the current calculations, the 0 K rotational barriers for isolated formamide and acetamide are predicted to be 14.2 and 12.5 kcal mol −1 , respectively.

Enthalpies of hydrogen bonds and proton affinities

Abstract: The heats of formation (Δ H HB 0 of hydrogen bonds involving neutral molecules of the OH ⋯ N and OH ⋯ O type are inversely correlated to Δ PA , the difference between the proton affinity of the O − anion and the proton affinity of the base. The difference between O( sp 2 ) and O( sp 3 ) bases is ascribed to protonation-rehybridization effects. The data are compared with those obtained for ionic hydrogen bonds. It is shown that in a wide range of Δ H HB 0 values (0-30 kcal mol −1 ) and Δ PA values (0-180 kcal mol − ), the Δ H HB 0 vs Δ PA correlation is not linear but there is no discontinuity between the weak and strong hydrogen bonds.

Dipole moments of indoles in their ground and the first excited singlet states

Abstract: The ground-state dipole moments of seven substituted indoles (3-acetylindole, 5- bromoindole, 3-formylindole, 2-indolecarboxylic acid, 5- indolecarboxylic acid, 5-methoxy-2-indolecarboxylic acid, and 5-nitroindole) have been measured at room temperature (298 K) in several solvents (benzene, dioxane, n-butyl ether). Their first excited singlet-state dipole moments have been determined by means of the equations of Bakhshiev and Chamma-Viallet, using the variation of the Stokes shift with the solvent dielectric constant-refractive index term. A comparison of the two sets of dipole moments indicates that the first excited singlet state of indoles is more polar than the ground state. The effect of substituents is discussed. The experimental values of dipole moments of selected indoles are compared with those calculated by the PPP (LCI-SCF-MO) method.

A molecular orbital study of the changes that accompany rotation of the HO group in phenol, and the barrier height

Abstract: The geometry, energy, dipole moment and total atomic charge distribution have been calculated for torsional angles of 0°, 30°, 60° and 90° with respect to rotation about the CO bond in phenol using the 6-31G basis set, assuming that the ring, the bonded O-atom, and the ring H-atoms all lie in the same plane, but otherwise with full geometry optimization. Additional calculations, using the 6-31G* (5D) basis set, showed very similar changes in geometry in going from the 0° (planar) to the 90° structure. Complete relaxation of the planarity constraint results in the ring taking up a very shallow boat-type conformation, o 7 C 1 ⋯C 4 = 1.4°, and a change in energy of only 0.05 kcal mol −1 . Evidence for an attractive interaction between the oxygen lone pair and the nearest ring hydrogens is discussed.

The influence of the basis set on the evaluation of conformational energies for small organic solutes in aqueous solutions

Abstract: Conformational energies of a small set of double rotor molecules (CH 3 CH 2 OH, CH 3 CHFOH(S), CH 2 FCH 2 OH, NH 2 CH 2 OH, CH 3 CH 2 OF) computed in vacuo and in aqueous solution with three basis sets (STO-3G, 3-21G, 4-31G) are compared in order to examine the basis set dependence of the results. Solvent effects are introduced with an SCF algorithm which relies on a continuum description of the solvent. The interaction operator depends on the description of the solute charge distribution and, consequently, on the basis set employed in the calculations.

Theoretical studies of the binding of methylamine and guanidine to carboxylate

Abstract: Ab-initio (Hartree—Fock) calculations with the 6-31G and 6–31* Gaussian basis sets are used to investigate the guanidinium ion — carboxylate ion interaction and the methyl-amine — carboxylate ion interaction. The calculations predict that the former is more stable as a zwitterion while the latter probably exists as a neutral—neutral complex in a solvent-free medium. It is also found that there is no barrier to the transfer of a proton from one species to the other.

Stacking interactions: Ab initio SCF and MP2 study on (H2O)2, (H2S)2, (HCN)2, (CH2O)2 and (C2H4)2

Abstract: The stacking complexes (H2O)2, (H2S)2, (HCN)2, (CH2O)2 and (C2H4)2 were studied at the SCF and MP2 levels with different basis sets. The SCF interaction energies are only slightly basis set dependent, with the exception of STO-3G, for which they are underestimated. MP2 interaction energies are, on the other hand, strongly basis set dependent. Minimal and split-valence basis sets give small values of this energy. A modified 6–31 G* basis set (diffuse polarization functions) provides reasonable values of SCF interaction energies, correlation interaction energies and total interaction energies of stacking complexes. The dipole—dipole electrostatic energy is comparable with the SCF interaction energy at large distances only. The dispersion energy agrees satisfactorily with the correlation interaction energy obtained with a basis set containing diffuse polarization functions.

Semiclassical models in theoretical chemistry. Some results and future prospects

Abstract: We present and justify the use of a set of related semiclassical models in the interpretation and prediction of a wide variety of molecular interaction phenomena. The semiclassical model uses ab initio quantum mechanical descriptions of the charge distribution of molecular subunits, interacting according to classical rules. The considered interaction phenomena mainly regard chemical processes (formation of bound aggregates, disruption and formation of covalent bonds) viewed under the energetic as well as the structural point of view, and solvent effects on the solute properties (conformation, reactivity, expectation value of physical observables), as examples of a more general interpretation scheme. Attention is paid to the transferability properties of subunits, and to the description of non-linear effects induced by the specific field acting on these subunits in the various situations. The paper summarizes results obtained in the preceding years, with the addition of fresh numerical data referred to the 3-21G basis set, and presents some new applications among which we signal a simplified procedure to compute the electrostatic contribution to the solvation free energy.

Ab initio study of structures and relative stabilities of RCP (R = H, F) and their energetically higher-lying isomers RPC

Abstract: Ab initio calculations at the levels of Moller—Plesset perturbation theory (MP4SDQ) and the configuration interaction (single and double excitations) method using polarized basis sets predict that both linear HPC and FPC are energy maxima with respect to the bending coordinate of HCP and FCP, respectively. HCP is the only minimum on its ground-state energy surface. For FCP, a second local minimum with a bend structure has been located and characterized. The barrier for fluorine rearrangement to the more stable isomer is somewhat small but large enough to allow the detection of the higher-lying isomer in an inert matrix at low temperature.

A study of the vibrational behavior of imidazole by the ab initio gradient method

Abstract: The complete harmonic vibrational force field of imidazole has been computed at the Hartree—Fock level using the 4–21 basis set of Gaussian orbitals. The harmonic constants were scaled by using scale factors previously derived by fitting the computed force field of benzene to the observed benzene vibrational spectrum. The resulting scaled force field was then used to predict the vibrational spectrum of imidazole. The NH wagging and NH stretching frequencies, which have no counterpart in benzene, were poorly predicted, but the mean deviation between experiment and prediction was only 9 cm−1 for the other in-plane (A′) vibrations and 27 cm−1 for the out-of-plane (A″) vibrations. In order to fit the NH modes and to obtain a better fit for the out-of-plane vibrations, a new set of scale factors, some of which turned out to be identical with those from benzene, was derived by fitting the computed spectrum of imidazole to the observed spectrum. This new set of scale factors was used to predict the vibrational spectra of several deuterated forms of imidazole (−1D, −3D, and −4D). The imidazole scale factors are presumably more accurate in scaling computed spectra of other 5-membered heterocyclic ring compounds. Dipole moment derivatives were also calculated and used to predict infrared intensities which are compared with experimental values. A few uncertainties in the experimental assignment of the imidazole spectrum can be clarified.

Ab initio structures and vibrational analysis of two planar configurations of 1,3,5-hexatriene

Abstract: The geometrical parameters of the tTt -hexatriene-1,3,5 and tCt -hexatriene-1,3,5 molecules have been computed at the RHF/6–31G level. The structures of both molecules are found to be planar in this approximation. The force fields of these molecules were calculated at the HF/6–31G//HF/6–31G level. The vibrational analyses of both molecules and their 2,3,4,5-tetradeuteroanalogues made it possible to refine the assignment of some experimental vibrational frequencies. The calculated mean amplitudes of vibrations of the non-deuterated molecules are compared with the experimental ones.

Recognizing kekuléan benzenoid molecules

Abstract: Two theorems about monotonous alternating paths in benzenoid hydrocarbons are formulated. They are used to derive algorithms for determining whether or not a given benzenoid possesses Kekule structures. Two efficient procedures are outlined. One of them is quite general and consists of systematic deletion of vertices.

Predicted rate constants for the exothermic reactions of ground state oxygen atoms and CH radicals

Abstract: A potential function has been derived for the ground-state surface of HCO which reproduces the spectroscopic properties of the equilibrium molecule and the results of ab-initio calculations at other stationary points on the surface. The potential has been used for a classical trajectory study of the vibrational excitation of CO on collision with fast H atoms and for a study of the reaction of ground-state oxygen atoms and CH radicals.

Ab initio MO calculations on structures and internal rotational barriers of nitroethene and nitrobenzene

Abstract: The geometry of nitroethene is optimized at the STO-3G, 4-21G, 4-31G, 6-31G and 6-31G** levels of molecular orbital theory. The geometry of nitrobenzene is optimized at the STO-3G level. These ground state geometries are compared with those deduced from microwave experiments. The potential curves for nitro group rotation have been calculated for nitroethene and nitrobenzene. For nitroethene the curve deduced from torsional frequencies in the far infrared is very well reproduced by the STO-3G calculations. Two-fold barriers obtained from the torsional frequency in nitrobenzene are smaller than the calculated barrier, possibly due to the presence of a negative four-fold component in the potential function.

Nonempirical modeling of the static and dynamic properties of the optimum environment for chemical reactions

Abstract: The previously proposed approach which allows nonempirical modeling of static properties of the optimal catalyst* has been extended to predict the optimal dynamics of the molecular environment of chemical reactions. This approach was applied here to characterize the optimal static and dynamic environment charge configuration for the reactions: HCN → HNC, HNC → HCN, BCN → BNC, BNC → BCN, F − + CH 3 F → FCH 3 + F − and CO 2 + H 2 O → H 2 CO 3 .

[1.1.1]Propellane, bicyclo[1.1.1]pentane and the effects of “inverted” carbons

Abstract: The stabilities and reactive properties of [1.1.1]propellane and bicyclo[1.1.1]pentane are analyzed by means of calculated electrostatic potentials and bond deviation indices. These were computed from ab initio SCF minimum-basis-set molecular wave functions, using geometries determined by optimization at the 6-31G* level. [1.1.1] Propellane has rather unusual regions of relatively strong negative electrostatic potential to the outsides of the bridgehead carbons, those commonly described as “inverted”. These carbons are viewed here as being somewhat unsaturated, with no direct bonding between them. They should be attractive sites for electrophilic attack, and it is suggested that they may also be reactive toward radicals. Bicyclo[1.1.1]pentane has a less strained structure, and the electrostatic potentials and bond deviation indices reveal the CC bonds to have some similarity to those in cubane.

Proton affinities of substituted cyanides

Abstract: Molecular orbital calculations at the ab initio STO-3G, 3–21G, 4–31G, and 6–31G* bases have been made for the relative proton affinities of an extended series of substituted cyanides, XCN. It is shown that geometry optimization at the 3–21G level is adequate and that 3–21G//3–21G calculations give very similar results to those obtained at the 6–31G*//3–21G level, except for X = Cl. Gas phase proton affinities are reported for many of the XCN compounds and these values are compared with the calculated results. Agreement between theory and experiment is excellent, except for some substituents which exert large field inductive effects.

Molecular complexes of hydrogen halides with ethers and sulphides studied by matrix isolation vibrational spectroscopy

Abstract: Infrared spectra are reported for mixtures of dimethyl ether, diethyl ether, hydrogen sulphide, dimethylsulphide and diethylsulphide with hydrogen chloride or hydrogen bromide in argon or nitrogen matrices. In addition to the bands due to the 1:1 complexes, absorptions due to 1:2 and 2:1 complexes were identified in many of the mixtures. The position and band shape of the perturbed HX stretching absorptions of the 1:1 complexes are discussed.

Analysis of the vibrational spectra of 1-methyluracil and its isotopic derivatives by ab initio calculations

Abstract: The vibrational spectrum of 1-methyluracil trapped in an argon matrix has been analysed based on ab initio Hartree—Fock SCF calculations with a split-valence 4–21 basis set. The directly computed theoretical harmonic force field was scaled with empirical scale factors which were transferred from uracil (except for the methyl vibrational modes) to provide an a priori prediction of fundamental frequencies and intensities. The average deviations between experiment and prediction were 9.8 cm−1 for the in-plane vibrations and 18.3 cm−1 for the ring out-of-plane modes. After a few corrections of assignment of the observed spectrum, a new set of scale factors was optimized to give the best force field available from combined consideration of the experimental and theoretical information. These scale factors reduced the average deviations to 6.7 cm−1 for the in-plane modes and to 11.7 cm−1 for the out-of-plane ones. The vibrational spectra of seven isotopic derivatives (-2180, -4180, -3d, -5d, -6d, -5, 6d2 and -CD3) of 1-methyluracil were predicted with the force field resulting from the optimized set of scale factors, and compared with the crystal-phase experimental data. A few misassignments in the experimental isotopic spectra have been corrected. Vibrational absorption intensities have been computed and compared with experiment and with an earlier calculation.

Theoretical considerations on the extended Boulton—Katritzky scheme

Abstract: It has been shown that in the rearrangement of 3-(2-aminoethyl)- and 3-(2-aminoaryl)-1,2,4-oxadiazoles the saturation of the side chain plays no decisive role from a mechanistic point of view, but the length of the side chain is the determining factor. The π-electronic system of the azole ring is also a prerequisite for a successful ring transformation. A linear correlation has been found between reactivity and CNDO/2 net charges on the N2 atom of the oxadiazole ring. The relative stabilities of the reacting species were assessed by CNDO/2 and ab initio geometry optimization calculations. The conformation of the side chains in two homologous molecules was analysed by X-ray diffraction.