Force field for in-plane vibrations of pyrazine

A general quadratic force field has been calculated using the MINDO/3 approximation for the in-plane vibrations of pyrazine. Analytical first derivatives and numerical second derivatives of the energy have been computed. The force constants have been refined to fit experimental frequencies of pyrazine-h₄, pyrazine-d₄ and cis-pyrazine-d₂.     

An improved general harmonic force field for ethylene

The recent availability of certain critical experimental data, both from this and other laboratories, for isotopic species of the ethylene molecule, has enabled the previously published GHFF to be redetermined with much greater precision. Several significant changes in the force constants occur, bringing the empirical values into better overall agreement with ab initio values. Calculated data using the new force field are considered to be sufficiently reliable to be used with confidence in spectroscopic and structural applications and are listed.     

Magnetic circular dichroism of the 3B3μ ← 1 Ag excitation in pyrazine

The magnetic circular dichroism spectrum of the lowest lying singlet—triplet transition of pyrazine is reported. The experimental data are in accord with theoretical predictions based upon the assumption of Hund's case (b) coupling in the ³B_3u state.     

A MINDO/3 MO study of oxidized flavins

The use of semi-empirical molecular orbital methods for the study of biologically important molecules has increased in recent years. The validity of well-known approximate Hartree-Fock molecular orbital methods such as MINDO/3 and MNDO still remains largely untested for biological molecules. Here we report its first application to the study of the electronic structures of isoalloxazines. Electronic charge distribution, variation of dipole moment, first ionization potential, electron affinity, orbital energies and electronic transitions are examined. Some of these results are compared with experiments. Good correlations with experiments were generally found in net atomic charge distributions, ionization potentials and electronic transitions. As a result of relatively good correlations with experiment application of the MINDO/3 method for extended study of biologically important coenzymes is promising.     

Treatment of hydrogen bonding within CNDO/2 and MINDO/3: CNDO/2H and MINDO/3H

A formalism has been developed to treat hydrogen-bonded A—H…?B systems within the CNDO /2 and the MINDO /3 methodologies. In this formalism the interactions are divided into three distinct classes; those between (a) two hydrogen-bonded atoms, (b) one hydrogen-bonded and non-hydrogen-bonded atom, and (c) two non-hydrogen-bonded atoms. The last class of interactions is treated solely by the existing CNDO /2 or MINDO /3 method. For A –H…?B systems, the core resonance integrals are individually parametrized depending upon the class of the interaction. Three types of A—H…?B systems have been thus far parametrized. Nine hydrogen-bonded dimers have been studied using the new formalism and the current CNDO /2 and the MINDO /3 methods. MINDO /3 predicts very large interatomic (A –B) distances for the equilibrium geometry, and relatively small stabilization values for the hydrogen-bond energies. CNDO/2 predicts the reverse. The new formalism for both CNDO /2 and MINDO /3 predicts accurate geometries as well as energies for all nine dimers. The new formalisms are called CNDO /2H and MINDO /3H. A general discussion of the nature of hydrogen bonding as exhibited by CNDO /2H and MINDO /3H is presented.     

Development and status of MINDO/3 and MNDO

The historical development of MINDO/3 and MNDO is outlined in relation to the parallel developments of the ab initio SCF approach. It is pointed out that both treatments are purely empirical so far as chemistry is concerned and complement one another, MINDO/3 and MNDO allowing calculations to be carried out when ab initio methods of comparable performance are inapplicable because they need one-thousand times more computing time.     

Ground states of molecules: Part XLII. Vibrational frequencies of isotopically-substituted molecules calculated using MINDO/3 force constants

The MINDO/3 method is used to calculate vibrational frequencies for ethylene, water and methylamine and their isotopic derivatives. The calculated ²H, ¹³C, ¹⁵N, and ¹⁸O vibrational shifts are in good agreement with the observed values, which suggests the method can be applied with confidence to the calculation of equilibrium and kinetic isotope effects in chemical reactions.     

Equilibrium structure and harmonic force field of the known PH3 and the unknown PH5

The equilibrium geometries and harmonic force fields of PH₃ and PH₅ are calculated in an ab initio way including electron correlation. The results for PH₃ are in very good agreement with experimental values, whereas those for PH₅ have to be regarded as predictions. We find for PH₅ in its equilibrium D _3h structure r _ax = 1.47 Å, r _eq = 1.42 Å and the harmonic vibration frequencies in Table 7 given under the heading “CEPA”. The barrier for Berry inversion is 2 kcal/mol. The ab initio calculation of phosphoranes such as PH₅ not only requires the inclusion of polarization functions (d on P and p on H) but is also very sensitive to the choice of these polarization functions. This problem is taken care of by a detailed comparison of various basis sets. It is confirmed that a (10/6) basis for P in “double zeta contraction” is better balanced than a (12/9) basis in “double zeta contraction” and that the total energy is not a good criterion for the quality of a basis.     

Evidence for Ag cluster vibrations in enhanced Raman scattering from the Ag/electrolyte interface

Low-frequency vibrational features are observed in the enhanced Raman spectra from an electrochemically roughened Ag electrode. Evidence is found that these features are related to resonant Raman scattering from the internal vibrations of small Ag clusters stabilized on the surface. Comparison of the experimental data with the results of a simple normal mode calculation suggests that a possible identity for such clusters is pyramidal Ag⁺₄. These clusters may be responsible for some of the “chemical” factor in SERS from coadsorbed molecules.     

Molecular mechanics. The MM3 force field for alkenes

The MM3 force field has been extended to include alkenes. Forty-five compounds were examined, and structures, conformational equilibria, heats of formation, and rotational barriers, were calculated. For a smaller representative group, the vibrational spectra and entropies were also calculated. In general, these quantities, except for the vibrational spectra, agree with available data to approximately within experimental error. The vibrational frequencies for a set of eight well-assigned structures were calculated to a root-mean-square error of 47 cm^?1.     

Calculation of harmonic force constants,vibrational frequencies and integrated intensities of some organic molecules using the MINDO—Forces method

The harmonic force constants, vibrational frequencies and integrated intensity ratios of CH₂, H₂O, CH₂O, C₂H₂, CO₂, HCN, CH₃, CH₄, and C₂H₄ have been calculated using the MINDO—FORCES program and the Pulay method for the calculation of the molecular force constants. The results obtained are in general quite satisfactory when compared with available literature values. The results are, however, not as satisfactory in case of molecules containing heteroatoms, due to the neglect of some dipolar repulsion integrals for the heteroatoms by the MINDO/3 method. Calculated integrated intensities for CH₃ and C₂H₄ agree well with experimental results. The calculated integrated intensities for other molecules are obtained for the first time and no comparison with published data is therefore possible.Part of the M.Sc. Thesis of K. H. A. 1978.     

The harmonic force field and absolute infrared intensities of butyne-2

The frequencies, harmonic force field and absolute IR intensities for butyne-2-d₀ and butyne-2-d₆ are reported. The final set of “harmonized” fundamental frequencies for butyne-2-d₀ and butyne-2-d₆ obeys the Teller—Redlich product rule very well. Starting values for the force constants were obtained from the harmonic force field of propyne, and diagonal force constants were adjusted in order to reproduce the experimental “harmonized” frequencies for the d₀ and d₆ compounds.The integrated IR intensities were measured according to the Wilson—Wells—Penner—Weber method, using nitrogen as a broadening gas. Thirteen sets of ?μ/?S values were obtained from the experimental intensities, using an iterative least-squares fitting procedure. This number could be reduced to one by use of several selection criteria. The signs of the remaining set appeared to be in complete agreement with the best set for propyne as reported both by Kondo and Koga and by Bode et al. The final ?μ/?S parameters were transformed into atomic polar tensors. Both kinds of intensity parameters are discussed and compared with corresponding parameters for related molecules.     

A MINDO/3 study of the hetero-Diels–Alder reaction

The transition states involved in the hetero-Diels–Alder reactions of acrylaldehyde–ethylene and butadiene–formaldehyde systems were characterized using an efficient method of locating a saddle point on the energy surface of two coordinates calculated with MINDO /3 method. It has been shown that parallel transition and rotation of the conjugate coordinate frame lead to the reaction and orthogonal coordinate system on the hyperbolic paraboloid energy surface. It was concluded that the method we adopted is similar to the “X-method” but is much simpler to apply to real reaction systems.     

Quantitative tests of mixed crystal excition theory. I. Naphthalene monomer1B2u and 3B1u spectra

A quantitative, computer processed spectroscopic study, using photon counting, on the first excited triplet and singlet states of dilute isotopic mixed crystals of naphthalene at 2 K is presented for C₁₀H₈; 1-DC₁₀H₇; 2-DC₁₀H₇; 1,4-D₂C₁₀H₆; 1,4,5-D₃C₁₀H₅; 1,4,5,8-D₄C₁₀H₄; 1,2,4,5,8-D₅C₁₀H₃; a β-D₄C₁₀H₄ and a β₂-D₆C₁₀H₂ as guests in C₁₀D₈ host crystals (and, for comparison, also for the same guests in a durene host crystal). The guest—host relative polarization Rashba formula has been verified quantitatively, and, as an added bonus, the elusive polarization ratio of the pure naphthalene crystal singlet Davydov components has been found to be 80 ± 20 (b/a), which is in poor agreement with the transition octupole—transition octupole model. The experimental guest energies and their concomitant quasiresonance shifts for bound singlet states (as well as the occurrences of unbound states) are in excellent quantitative agreement (about 1 cm^?1) with those calculated using a Green's function formalism based on the ideal mixed crystal approximation and on a restricted Frenkel type dispersion relation derived from resonance pairs. The same Green's function also accounts quantitatively (within 10%) for the guest singlet state exciton localizations (guest excitation amplitudes). The triplet exciton state reveals an orientational site splitting (about 0.7 cm^?1) for the 0—0 transition of the I-DC₁₀H₇ guest in C₁₀D₈ host. The order of the α and β substituted deuteronaphthalenes in the triplet state is reversed from that of the singlet state. The last two observations are related to the different nature of the lowest Π-Π^* singlet and triplet states of naphthalene.     

Modified MINDO/3 13C chemical shift calculations for simple hydrocarbons

Calculations of ¹³C chemical shifts in some simple hydrocarbons have been carried out using the GIAO approach in the MINDO/3 semiempirical formalism. In order to achieve reasonable agreement with experiment it is necessary to modify (increase) the vacant orbital energies in the MINDO/3 calculation in order to reduce the magnitude of the paramagnetic contribution, and to also modify this dominant term by generally reducing it as a function of the number of hydrogen and carbon atoms bonded to the resonant nucleus in question. For a set of 34 resonant nuclei of the simpler hydrocarbons, agreement with experiment of the order of 7.8 ppm is attained; however, pathological cases such as cyclopropane and some simple allenes continue to cause problems, increasing the standard deviation of the full set to 12.5 ppm. Our results indicate that the MINDO/3 approach is as viable for ¹³C chemical shift calculations as other semiempirical approaches, all of which seem currently to be limited to a standard deviation of the order of 10 ppm.     

The vibrational force field of halogenated aromatic molecules: II. Out-of-plane vibrations of fluoroaromatics

A general quadratic force field is determined for the out-of-plane vibrations of florobenzenes. The force constants for the out-of-plane wagging of both CH and CF bonds decrease by about 10 % for each ortho fluorine substituent. No significant meta dependence could be detected. It is shown that the present field is in excellent accord with the chlorobenzene field of Scherer and with the benzene fields of Whiffen and of Kydd when due allowance is made for differences in coordinate definitions.     

Evaluation of MINDO/3 calculated structures. II. Branching errors in alkanes and cycloalkanes

MINDO/3 calculations have been carried out for a series of branched chain alkanes in order to assess effects of branching on calculated geometries and heats of formation (ΔH_f). With vicinal branching, MINDO/3 calculates the central C? C bond to be too long. Bond angles are also found to be distorted. Errors in calculated heats of formation are large when geminal branching is present and significant with vicinal branching. Branching error corrections for ΔH_f have been derived and applied to a separate series of branched acyclic and cyclic compounds. For the test sample, application of the branching error corrections gave calculated structures of acyclic branched hydrocarbons with heats of formation having an average absolute error of 1.3 kcal/mole rather than 17.3 kcal/mole before correction. Cyclic branched hydrocarbons are shown to be less well corrected. Calculations of heats of reaction have also been carried out for some isomerization and cyclization reactions using the MINDO/3 and MNDO methods. It is clear from the comparisons that MNDO calculations give less severe errors for highly branched compounds but the errors are still substantial. For prediction of heats of reaction, the error-corrected calculations are shown to be superior to the “raw” calculations obtained by MINDO/3 or MNDO.