Vibrational resonances in infrared spectra of uracils

Two regions in the i.r. spectra of uracils are analyzed: (a) the NH stretching region in crystals, and (b) the CO stretching region in matrix-isolated monomers. In these regions there is clear evidence of resonances leading to energy splittings and re-distributions of intensifies. These are: (a) a harmonic resonance between the NH stretching in hydrogen-bonded groups and the CH stretching, and (b) an anharmonic (Fermi) resonance between the CO stretching and a combination band involving the N₃H and CO bending vibrations. A theoretical interpretation of the resonances is presented.     

Comparative ab initio and valence electron study of unsaturated 3-membered ring systems. Questions of conjugative and inductive group abilities,of

The method of conjugative interruption in conjunction with the CNDO/S and an ab initio method is applied to cyclopropene, cyclopropenone, cyclopropenthione, thiirene 1-oxide, and thiirene 1,1-dioxide. Thereby the orbital interactions between the π orbitals of the CC and M(MCH₂, CO, CS, SO, and SO₂) subunits a quantitatively analyzed as well as their consequences as regards the conjugative and inductive abilities of M, the aromaticities, and geometries of this chemically important series of compounds. A detailed comparison of the CNDO/S and ab initio results is made. Both sorts of calculation yield surprisingly well concurring results. Where the results of both methods basically differ (as to the sign of the aromaticities, or the splitting pattern of two interacting orbitals) it can be traced back to the zero differential overlap approximation made for the valence electron procedure. In addition, the paper presents the first ab initio data on cyclopropenethione, thiirene 1-oxide, and thiirene 1,1-dioxide.     

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

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) Å for normal or unstrained hydrocarbons; ?0.011(3) to ?0.016(3) Å for cyclobutane type compounds; and +0.001(5) to +0.004(4) Å for CH₃ conjugated with CO. For CO single bonds the ranges are ?0.006(9) to +0.002(3) Å for CO conjugated with CO; and ?0.019(3) to ?0.027(9) Å 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) Å and in CH₃OCH₃ and C₂H₅OCH₃ the r_s — r_e differences are ?0.029(5), ?0.040(10) and ?0.025(10) Å. 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CH₃ corrections are from +0.005(6) to ?0.006(6) and from ?0.009(2) to ?0.014(6) Å; and the range for NCO is +0.012(3) to +0.028(4) Å. For isolated CC double bonds the range is + 0.025(2) to +0.028(2) Å. For conjugated CC double bonds the correction is less positive (+0.014(1) Å for benzene). For CO double bonds the corrections are ?0.004(3) to +0.003(3) Å. 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)°.     

The structure of propadienone (CH2CCO)

A complete r_o structure has been obtained for propadienone (CH₂CCCO) with the aid of ab initio molecular-orbital calculations. The effect of electron correlation has been investigated using third order Møller-Plesset perturbation theory. The molecule is found to be planar bent (CCC145°) and the calculated structure yields rotational constants which are in good agreemant with experimental values.     

Inorganic Syntheses,Volume XXI : edited by J.P. Fackler,Jr., Wiley-Interscience,New York, 1982, xvii + 215 pages, £29.25.

The 1/1 reaction of Et₂Zn with N-t-butyliminopropanone (t-BuNC(H)C(Me)O) leads to quantitative formation of dinuclear [EtZ$\text{n(Et)(t-Bu)NC(H)=C(Me)O}$]₂ via ethyl transfer within the unstable Et₂Zn(t-BuNC(H)?C(Me)O) complex. An X-ray structure determination has shown the product to have a dinuclear structure involving a N,O-chelate bonded [Et(t-Bu)NC(H)C(Me)O]^? mono-anionic ligand and a central four membered Zn₂O₂ ring formed by intermolecular ZnO coordination. Hydrolysis of this zinc complex gives a quantitative yield of N-t-butyl-N-ethylamino propanone, which upon reaction with Et₂Zn reforms the dinuclear zinc complex.     

Raman intensities of liquids: Absolute scattering activities and electro-optical parameters of the halate ions ClO3−, BrO3−, and IO3− in aqueous solutions

Absolute Raman scattering activities of aqueous solutions of sodium bromate and lithium iodate have been measured against NaClO₄ as an external standard. Electro-optical parameters (EOPs) for the BrO and IO bonds were calculated. Equilibrium bond polarizabilities were estimated from refractive index measurements in connection with Raman intensities of the bending modes. Relations between Bragg—Slater radii and EOPs are discussed. EOPS calculated from experimental data are compared with those from ab initio calculations.     

Ab initio quantum mechanical calculation of vibrational frequencies,infrared intensities and equilibrium geometry of the allyl radical

Ab initio and semi-empirical quantum mechanical calculations have been performed to assist the interpretation of i.r. spectra of the allyl radical. After scaling, most calculated frequencies fit the observed frequencies with a deviation of less than 1%. The present assignment is substantially different from earlier assignments. Misfits between measured and calculated geometry indicates that the experimental CC bond lengths are too long, while the experimental CH distances are too short.     

Scaled quantum mechanical (SQM) force field and theoretical vibrational spectrum for benzonitrile

The complete harmonic force field of benzonitrile has been determined by ab initio Hartree—Fock calculations using a 4–21 Gaussian basis set. As force constants are systematically over-estimated at this level, the directly calculated force field was scaled by empirical factors previously optimized for benzene and HCN. Frequencies calculated from this scaled quantum mechanical (SQM) force field confirm the published experimental assignments for benzonitrile, benzonitrile-p-d and benzonitrile-d₅. Aside from the CH (and CD) stretching frequencies, which are strongly affected by anharmonicity, the mean deviation between the observed and calculated frequencies is below 9 cm⁻¹ for each isotopomer. Theoretical i.r. intensities reproduce the main features of the spectra semiquantitatively.     

Darstellung, eigenschaften und kristallstruktur von methylgalliumdiacetat CH3Ga(OOCCH3)2

Trimethylgallium reacts with acetic acid in a $\text{1}\text{2}$ molar ratio yielding methylgallium diacetate, CH₃Ga(OOCCH₃)_2? The structure is determined by vibrational spectroscopy and the crystal structure is described. Methylgallium diacetate crystallizes in the monoclinic space group P2₁/c with lattice constants a 776.5, b 1428.9, c 1406.3 pm, β 91.87° and eight formula units per cell. The monomers are linked together by acetate groups forming polymeric, waved layers. Besides the bridging acetate there are also “free” acetate groups coordinated at the distorted trigonal-bipyramidal coordinated Ga(1) atom. A second gallium atom Ga(2) is coordinated distorted tetrahedrally, the acetate groups bonded to Ga(2) being all bridging. The mean intermolecular distances are: GaC 194.6 pm, Ga(1)O_apical 215.3 pm, Ga(1)O_eq(bridge) 194.3 pm, Ga(1)O_eq(free) 187.3 pm, Ga(2)O 191.3 pm, CC 151.5 pm, CO 119.4 pm, CO 131.1 pm, CO 127.3 pm.     

Thermodynamics of vinyl ethers—XVII : Thermodynamic stability of 1,2-dialkoxyethylenes

The relative thermodynamic stability of the monoalkoxy- and 1,2-dialkoxyethylene systems [OCCH(C) and OCCO, respectively] has been studied by chemical equilibration of suitable isomeric compounds. Although a single alkoxy substituent stabilizes the CC bond by about 25 kJ mol^?1, the 1,2-dialkoxyethylene system is no more stable than the monoalkoxyethylene (“ordinary” vinyl ether) system. On the contrary, the MeOCCOMe system was found to be about 4 kJ mol^?1 (on an enthalpy basis) less stable than the system MeOCCH.     

Sensitivity of the H + CH3 → CH4 recombination rate constant to the shape of the CH stretching potential

Using a potential-energy surface obtained in part from ab initio calculations, the H + CH₃ → CH₄ bimolecular rate constant at T = 300 K is determined from a Monte Carlo classical trajectory study. Representing the CH stretching potential with a standard Morse function instead ofthe ab initio curve increases the calculated rate constant by an order of magnitude. The experimental recombination rate constant is intermediate of the rate constants calculated with the Morse and ab initio stretching potentials.Two properties of the H + CH₃ α CH₄ potential-energy surface which significantly affect the recombination rate constant are the shape of the CH stretching potential and the attenuation of the H₃CH bending frequencies. Ab initio calculations with a hierarchy of basis sets and treatment of electron correlation indicate the latter is properly described [13]. The exact shape of the CH stretching potential is not delineated by the ab initio calculations, since the ab initio calculations are not converged for bond lengths of 2.0–3.0 Å [12]. However, the form of this stretching potential deduced from the highest-level ab initio calculations, and fit analytically by eq. (2), is significantly different from a Morse function. The experimental recombination rate constant is intermediate of the rate constants calculated with the Morse and ab initio CH stretching potentials. This indicates that the actual CH potential energy curve lies between the Morse and ab initio curves. This is consistent with the finding that potential energy curves for diatomics are not well described by a Morse function [12].     

An ab initio SCF and CI study of ketene imine

The electronic structures of the ground and excited states of ketene imine (_H^H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.     

Infrared studies of conformational isomerism in some gem-chloronitrosoalkanes

The IR spectra of 2-chloro-2-nitrosopropane, 2-chloro-2-nitrosobutane and 2-chloro-3-methyl-2-nitrosobutane have been studied in the gas phase and solid phase, and in solutions of varying polarity. The spectra reveal clear evidence for the hindered internal rotation of the NO functional group about the CN bond in a splitting of the NO stretching band. The spectra can be interpreted as arising from energetically favoured cis conformational isomers, exhibiting the higher of the observed NO stretching frequencies. In each case, assignment of the lower NO stretching frequency is uncertain on the basis of experimental evidence. The results of semi-empirical molecular orbital calculations, however, support the assignment of this feature to the gauche conformational isomers. Temperature dependent IR spectroscopy has been employed in the determination of conformational enthalpy differences in both the gas phase and non-polar solution, which are then compared to the results of semi-empirical molecular orbital calculations.     

Ab initio studies of structural features not easily amenable to experiment: Part 6. Quantitative estimate of the effect of bond delocalization on structure and hyperconjugative interaction of the amide group

The structural changes, which occur in the amide unit when the NH₂-group is twisted out of plane by rotation about the NC bond, have been determined by comparing the completely relaxed ab initio geometries of planar and perpendicular formamide and acetamide. In the perpendicular conformation, in which the π-electron amide resonance is uncoupled, the NC bond distance is 0.080.09 Å longer than in the planar form; the CO bond distance is about 0.01 Å shorter; NH distances are about 0.01 Å longer; and HNC angles are 510° smaller, whereas the CNO angle is relatively constant. Because of the apparent invariance of CH₃-hyperconjugation effects in planar and perpendicular acetamide, it is tentatively postulated that anomeric orbital interactive effects (involving the lone pair on NH, the CO π-electron pair and antibonding π*-group-orbitals on C(α) in NHC(HR)C(O)), which should be an important factor in determining peptide chain conformation, do not vary significantly with small deviations from amide group planarity.     

Study of the excited states of HNO and its emission spectrum in hydrogen-based flames

INDO and configuration-interaction calculations of lower singly and doubly excited states of the gaseous HNO molecule are reported. An analysis of the HNO emission spectrum in a cool flame is also carried out. A clear symmetrical relationship among the energies of pairs of bands in the spectrum is detected, and the bands are identified as emissions from the lowest singly excited electronic state (n, π^*) invovling NO stretching and HNO bending modes of vibrations and their combinations. In addition, a single band at 756 nm appeared not to be emitted from the same electronic state. Possible origins of this band are suggested.     

The conformations of allylic alcohols in solution from FT-i.r. OH stretching vibration measurements

The distribution of conformations of allylic alcohols in CCl₄ differs from that in the vapour phase and from ab initio calculations. FT-i.r. measurements of the OH stretching vibrations show two peak maxima which can be resolved by band splitting techniques. The predominant conformations of allylic alcohols are intramolecularly hydrogen bonded, with a conformation gauche (G or G′) with respect to rotation about the CO bond and eclipsed (E or E′) with respect to rotation about the CC bond. In contrast with vapour phase data and ab initio calculations, no other hydrogen bonded conformations have been identified.For primary alcohols, the shoulder to the higher wavenumber side can be unequivocally assigned to conformations in which the OH is free, with a trans (T) conformation with respect to rotation about the CO bond.The secondary allylic alcohols exhibit no bands that can be attributed to free OH conformations except in the case of severe steric interaction in the eclipsed (E) conformation.In accordance with our previous work on solvent interactions with the OH group, the contrast with the vapour state assignments are interpreted in terms of an interaction of the solvent with the OH proton which destabilizes the hydrogen bond.     

Application of the Seth-Paul-Van Duyse equation—III : Transmission of polar effects by the furan ring

The improved Seth-Paul-Van Duyse equation (SPVDE) has been used to determine transmissive factors of polar effects for the furan ring. The SPVDE was applied to the CO stretching frequencies of OO-trans and OO-cis conformers of a series of substituted 5-phenyl-2-furancarbonyl compounds measured in carbon tetrachloride. The transmissive factors for furan ring in both conformation OO-trans and OO-cis have been calculated with a higher accuracy than found by using the methods described earlier. The applicability of the improved SPVDE to the 96 CO stretching frequencies of various 5-phenyl-2-furancarbonyl compounds has been proved.     

The structure of the H3O+ (hydronium) ion

Near Hartree-Fock level ab initio molecular orbital calculations on H₃O⁺ and a minimum energy structure with θ(HOH) = 112.5° and r(OH) = 0.963 Å and an inversion barrier of 1.9 kcal/mole. By comparing these results to calculations on NH₃ and H₂O, where precise experimental geometries are known, we estimate the “true” geometry of isolated H₃O⁺ to have a structure with θ(HOH) = 110-112°, r(OH) = 0.97–0.98 Å and an inversion barrier of 2–3 kcal/mole. Our prediction for the proton affinity of water is ≈ 170 kcal/mole, which is somewhat smaller than the currently accepted value.     

Solvatation preferentielle dans les systemes ternaires polyvinylpyrrolidone/solvant halogene/solvant donneur de proton—II: Etude,par spectroscopie infra-rouge,des interactions hydrogene

We have studied, for dilute solutions of polyvinylpyrrolidone in halogenated solvent/protondonor solvent mixtures, the spectroscopic perturbations of the stretching vibration band v(CO) of the polymer and those of the stretching vibration band v(OH) in the particular case of trifluoroethanol. These measurements indicate a hydrogen-bond association polymer/proton-donor solvent, increasing with the proton-donor concentration in the mixture. The interaction with solvents such as trifluoroethanol and phenol is much stronger than that due to ethanol. Hydrogen-bond complexes are of $\text{1}\text{1}$ type. However, study of the v(CO) and v(OH) bands leads, in the case of trifluoroethanol, to the proposal of a complex of $\text{1}\text{2}$ type the structure of which is suggested.     

On the transferability of conformation in FC(O)S-containing compounds: Conformation and properties of methylfluorocarbonyl disulphide FC(O)SSCH3

Methylfluorocarbonyl disulphide, FC(O)SSCH₃, was prepared for the first time by reaction of FC(O)SCl with CH₃SH at room temperature. Infrared data for the vapour and matrices (Ar, Ne and N₂) as well as Raman, UV, mass and ¹⁹F, ¹³C and ¹H NMR spectra have been obtained and interpreted.From these data, the most stable conformer was deduced to have the gauche conformation with respect to the FC(O) and CH₃ groups with the syn conformation between the CO and SS bonds having C₁ molecular symmetry. This conformer is in equilibrium with another, possibly the corresponding anti, referring to the CO and SS bonds.The main structure found for FC(O)S-containing compounds seems to be the syn conformation.