László Harsányi

The molecular structure of uracil: an electron diffraction study

Abstract The molecular structure of free uracil has been determined by electron diffraction. Planar or nearly planar models fit the experimental data. Differences between the four CN bond lengths as well as those between the two CO bond lengths were assumed from ab initio (4–21) calculations in the electron diffraction analysis. The following bond lengths (rg) and bond angles (rα) were determined; the estimated total errors are parenthesized as expressed in units of the last digit: r(CN)mean 1.399(6), r(CC) 1.462(8), r(CC) 1.343(24), r(CO)mean 1.212(3) A, C2N1C6 123.2(12), N3C4C5 115.5(18), C4C5C6 119.7(21), C5C6N1 122.1(22), N1C2O8 123.8(14), C5C4O10 124.3(20)°. The structural parameters by electron diffraction are in agreement with those by quantum chemical optimizations (ab initio 4–21, MINDO/3). The bond lengths indicate partial delocalization in the ring.

Infrared levels of monomeric uracil in cryogenic matrices

Abstract The infrared spectrum of monomeric uracil, trapped in Ar and N 2 cryogenic matrices, is presented and compared with the infrared and Raman spectra recorded for this same compound in associated forms, as observed in its pure solid state and in solution. The monomeric molecules are all in the 2,4-diketotautomeric form and are planar, as can be deduced indirectly from the absence of band multiplicity due to conformeric equilibrium. The assignment is based on comparison with previous works on uracil and with matrix isolation studies on other cis - and trans -imides and is supported by spectral comparison with N,N -dideuterouracil trapped in the same cryogenic matrices. With few exceptions, the assignment is consistent with that obtained by theoretical approaches. On passing from an Ar to an N 2 matrix, the bands due to νNH and γNH shift to lower and higher wavenumbers, respectively, as a result of a specific interaction between the NH groups and the N 2 molecules.

Calculation of the in-plane force-constants and vibrational-spectra of pyridine and its deuterated derivatives by the CNDO-2 force method

Abstract The in-plane vibrations of pyridine and its deuterated derivatives (4-d1, 2,6-d2, 3,5-d2, and d5) have been studied with the CNDO/2 force method. The calculated values of the force field scaled by five empirical parameters yield good agreement with the experimental spectra. The results seem to permit the reassignment of some bands.

EQUILIBRIUM GEOMETRIES OF URACIL AND ITS C- AND N-METHYLATED DERIVATIVES

Abstract Equilibrium geometries, heats of formation and dipole moments of free uracil (U), 1-methyl-uracil (1-MeU), 3-methyl-uracil (3-MeU), 5-methyl-uracil (5-MeU, thymine), 6-methyl-uracil (6-MeU), 1,3-dimethyl-uracil (1,3-diMeU) and 5,6-dimethyl-uracil (5,6-diMeU) have been determined by semiempirical MINDO/3 calculations using Pulays force method. Monosubstitutions by the methyl group manifested themselves in mainly local changes in bond distances and angles, while effects of disubstitutions, in good approximation, proved to be additive. Additivity also holds for heats of formation and dipole moments.

The molecular structure of maleimide: an electron diffraction study

Abstract The molecular structure of maleimide has been investigated by electron diffraction. The experimental data are consistent with a C 2v symmetry model. The following bond lengths ( r g ) and bond angles ( r c ) were determined:NC 1.409 = 0.003, CC 1.508 ± 0.003, C=C 1.344 ± 0.004, and C=O 1.206 ± 0.002 A; CNC 112.0 = 0.2, NCC 106.8 ± 0.2, NC=O 123.9 ± 0.3, and CCH 114.7 ± 1.3°. The electron diffraction structure is in agreement with the results of quantum chemical calculations. The bond lengths indicate a somewhat greater delocalization about the N atom than in the OCCCCO skeleton.

Scaled quantum mechanical (SQM) ab initio force field and vibrational spectra of maleimide (1H-pyrrole-2,5-dione)

Abstract The vibrational spectrum of free maleimide has been interpreted using a theoretical general valence force field (GVFF) calculated ab initio at the Hartree-Fock level using the 4–21 split valence basis set. The directly computed GVFF was scaled with 14 empirical factors transferred from other molecules. The vibrational fundamentals predicted by this scaled GVFF enabled corrections to be suggested for several previous empirical assignments. The scale factors were also optimized directly on the experimental spectrum of maleimide producing the best GVFF obtainable from combined experimental and theoretical evidence. This scaled GVFF yielded frequencies agreeing with experimental values and with mean deviations of 6.8 and 9.6 cm−1 for in-plane and out-of-plane frequencies, respectively. The vibrational frequencies of the 2H and 15N isotopomers of C2v symmetry are also presented and discussed.

Theoretical force field and vibrational spectrum of maleimide by the CNDO/2 force method

Abstract In-plane and out-of-plane vibrations of maleimide have been studied by the CNDO/2 force method. To account for systematic errors, the direct theoretical force field was scaled by empirical factors optimized on the basis of experimental frequencies. The calculations suggest some new assignments in the vibrational frequencies.

Studies of UV photochemistry of psoralen and angelicin by infrared spectroscopy

Abstract The UV homodimerization reaction of psoralen and angelicin in crystalline thin layers has been investigated by means of transmission infrared and infrared ATR (attenuated total reflection) spectroscopy. In the case of psoralen layers isoorientation was found. Dichroic ratios for several vibrational bands have been obtained. After irradiation, cis-syn photodimers were found for both psoralen and angelicin.

Studies of UV photochemistry and transition moment directions of 4,5′,8-trimethyl-psoralen (TMP) by means of polarized infrared ATR spectroscopy

Abstract The UV homodimerization of 4,5′,8-trimethyl-psoralen (TMP) both in a solvent and in crystalline thin layers has been investigated. In the crystalline phase, TMP was found to dimerize completely to the trans—anti stereoisomer. On the basis of polarized IR ATR spectra of thin layers deposited onto a KRS-5 plate, both the monomer and the dimer of TMP were found to have well-defined orientations. The directions of several transition moments have been calculated. On irradiating TMP in solution, the photochemical reaction results in the cis—syn stereoisomer.

Electronic structure and photochemical reactivity of nucleic acid constituents: I. Uracil derivatives

Abstract The difference in the photochemical reactivity of uracil, 5-aza-uracil and 6-azauracil was studied by quantum chemical methods. The equilibrium geometries of monomers and that of uracil dimer were calculated by the CNDO/2 method. The results showed a somewhat larger dihedral angle compared to existing X-ray diffraction data. The ground and excited state energies of the monomers were calculated by the perturbational PPP method at different points along a simplified reaction path. Agreement or calculated and spectroscopic data in thin layers is discussed.