Vibrational Study of Alkyl Isocyanates in Solution

1992 ◽  
Vol 46 (6) ◽  
pp. 972-980 ◽  
Author(s):  
Richard A. Nyquist ◽  
Davin A. Luoma ◽  
Curt L. Putzig
Keyword(s):  
Carbon Atom ◽  
High Frequency ◽  
Fermi Resonance ◽  
Hydrogen Atoms ◽  
Tert Butyl ◽  
Vibrational Study ◽  
Steric Factors ◽  
Recorded Data ◽  
Ir Bands ◽  
Methyl Analog

The vasym. NCO frequencies for alkyl isocyanates occur at higher frequency in CHCl3 solution than in CCl4 solution. With the exception of tert-butyl isocyanate, the vasym. NCO mode increases in frequency as the mole % CHCl3/CCl4 increases. The vasym. NCO mode for tert-butyl analog increases in frequency up to a certain mole % CHCl3/CCl4 and then vasym. NCO decreases in frequency. The vasym. NCO mode for n-butyl isocyanate occurs at an exceptionally high frequency for the alkyl isocyanate studied, and this result is explained in terms of the formation of a pseudo six-membered intermolecular hydrogen-bonded ring. Inductive and steric factors also influence the type and form of solvent/solute complexes formed vs. mole % CHCl3/CCl4 as determined by study of the vasym. NCC frequencies. In general, the vasym. NCO and the vsym. NCO modes for alky isocyanates decrease in frequency as the number of hydrogen atoms on the α-carbon atom of the alkyl group decreases from 3, to 2, to 1, to 0. Some of the alkyl isocyanates exhibit two significant IR bands in the region expected for vasym. NCO, and the methyl analog exhibits three significant bands in this region of the spectrum. These IR bands are the result of vasym. NCO in Fermi resonance with combination tones, and the unperturbed frequencies have been calculated with the use of the recorded data.

Infrared Study of Alkyl Isothiocyanates in Carbon Tetrachloride and/or Chloroform Solutions

1993 ◽  
Vol 47 (6) ◽  
pp. 677-686 ◽  
Author(s):  
R. A. Nyquist ◽  
C. W. Puehl
Keyword(s):  
Carbon Tetrachloride ◽  
Alkyl Group ◽  
High Frequency ◽  
Fermi Resonance ◽  
Methyl Ethyl ◽  
Infrared Study ◽  
Tert Butyl ◽  
Electron Donation ◽  
Hydrogen Bonded

The νasym.NCS frequencies for alkyl isothiocyanates occur at higher frequency in CHCl3 or CDCl3 solution than in CCl4 solution. The νasym.NCS mode increases in frequency as the mole % CHCl3/CCl4 increases. The νasym.NCS mode for butyl isothiocyanate occurs at an exceptionally high frequency, and this result is explained in terms of the formation of a pseudo-six-membered intramolecularly hydrogen-bonded ring. The νasym.NCS modes for the alkyl isothiocyanates are corrected for Fermi resonance (FR), with the exception of the propyl analog. The propyl analog appears to have three other modes in FR with νasym.NCS, and an equation has not yet been developed to correct for FR in this case. The unperturbed νasym.NCS frequencies for alkyl isothiocyanates decrease in the alkyl order: methyl, butyl, ethyl, and tert-butyl. The decrease in frequency of νasym.NCS in the order methyl, ethyl, and tert-butyl isothiocyanate is attributed to weakening of the N=C=S bonds due to σ electron donation of each alkyl group. In the case of the butyl analog, the σ electron donation is partially compensated for by the postulated existence of a pseudo-six-membered hydrogen-bonded ring.

Mechanism and stereochemistry of enzymic reactions involved in porphyrin biosynthesis

1976 ◽  
Vol 273 (924) ◽  
pp. 117-136 ◽  
Keyword(s):  
Schiff Base ◽  
Hydrogen Atom ◽  
Carbon Atom ◽  
Pyridoxal Phosphate ◽  
Hydrogen Atoms ◽  
Bond Forming ◽  
Aminolaevulinic Acid

5-Aminolaevulinate synthetase catalyses the condensation of glycine and succinyl-CoA to give 5-aminolaevulinic acid. At least two broad pathways may be considered for the initial C—C bond forming step in the reaction. In pathway A the Schiff base of glycine and enzyme bound pyridoxal phosphate ( a ) undergoes decarboxylation to give the carbanion ( b ) which then condenses with succinyl-CoA with the retention of both the original C2 hydrogen atoms of glycine. In pathway B, loss of a C2 hydrogen atom gives another type of carbanion ( c ) that reacts with succinyl-CoA. Evidence has been presented to show that the initial C—C bond forming event occurs via pathway B which involves the removal of the pro R hydrogen atom of glycine. Subsequent mechanistic and stereochemical events occurring at the carbon atom destined to become C5 of 5-aminolaevulinate have also been delineated.

Vibrational study and spectra-structure correlations in magnesium disaccharinate heptahydrate, Mg(sac)2⋅7H2O

2008 ◽  
Vol 27 (1) ◽  
pp. 1
Author(s):  
Gligor Jovanovski ◽  
Petre Makreski ◽  
Bojan Šoptrajanov
Keyword(s):  
Experimental Data ◽  
Raman Spectrum ◽  
The Other ◽  
Water Molecules ◽  
Carbonyl Groups ◽  
Structural Units ◽  
Vibrational Study ◽  
Structure Correlations ◽  
Stretching Vibrations ◽  
Ir Bands

Infrared and Raman vibrational spectra of magnesium disaccharinate heptahydrate, Mg(sac)2⋅7H2O, in the 4000–380 cm–1 region (for infrared) and 4000–100 cm–1 region (for Raman) were studied. The assignment of the spectra was based on the experimental data for the previously studied metal saccharinates as well as the literature data for the ab initio calculations on the free deprotonated saccharinato species. Special attention was paid to the analysis of the H2O, CO and SO2 stretching modes. The spectral picture in the regions of the water, carbonyl and sulfonyl stretches is correlated with the number of the crystallographically determined non-equivalent H2O, CO and SO2 structural units. It was found that the presence of seven crystallographically different water molecules in the structure (fourteen different Ow⋅⋅⋅O and Ow⋅⋅⋅N distances) is not reflected in the appearance of the expected fourteen IR bands in the region of the OD stretching vibrations of the isotopically isolated HDO molecules. This must be due to the existence in the structure of several Ow⋅⋅⋅O or Ow⋅⋅⋅N hydrogen bonds with very similar strengths causing an overlap of the corresponding bands in the spectrum. Despite the presence of two carbonyl groups with practically identical C–O distances [124.2(3) and 124.0(3) pm], two clearly separated bands are registered in the carbonyl stretching region of the IR (1660 and 1627 cm–1) and Raman spectrum (1648 and 1620 cm–1). On the other hand, although two nonequivalent SO2 groups are present in the structure of Mg(sac)2⋅7H2O, only one pair of bands due to SO2 stretchings [νas(SO2 and νs(SO2) modes] is registered in the IR spectrum.

A theoretical and experimental investigation of some unusual intermolecular hydrogen-bond IR bands — Appearances can be deceptive

2006 ◽  
Vol 84 (10) ◽  
pp. 1371-1379 ◽  
Author(s):  
Grzegorz Litwinienko ◽  
Gino A DiLabio ◽  
K U Ingold
Keyword(s):  
Hydrogen Bond ◽  
Complex Formation ◽  
Ethylene Oxide ◽  
Potential Energy ◽  
Potential Energy Surfaces ◽  
Intermolecular Hydrogen Bond ◽  
Phenyl Group ◽  
Fermi Resonance ◽  
Energy Surfaces ◽  
Ir Bands

The IR spectra of the O-H stretch for hydrogen bonds (HBs) arising from complex formation between the HB donor (HBD), 4-fluorophenol, and the HB acceptors, peroxides and ethers, frequently show asymmetry that appears to arise from two incompletely resolved bands from two different complexes, but the O-H HB bands with the HBD methanol are symmetric (M. Berthelot, F. Bessau, and C. Laurence. Eur. J. Org. Chem. 925 (1998)). The present studies show that this difference in O-H HB band shapes also is true for other phenols and alcohols. However with ethylene oxide, 4-fluorophenol gives an almost symmetric O-H HB band with a very broad maximum, while alcohols give symmetric O-H HB bands with well-defined maxima. It is shown by experiment that the unusual O-H HB band shapes for the phenols are not due to Fermi resonance and are unrelated to the enthalpies of HB complex formation. Theoretical exploration of the potential energy (PE) surfaces for complexes of 4-fluorophenol and methanol with tert-butyl methyl ether and ethylene oxide reveals that O-H HB band asymmetry or broadness cannot be ascribed to the presence of two different HB complexes. For this ether, the PE surfaces for rotation about the HB and for up-and-down motion of the HBD with respect to the COC plane of the ether are relatively symmetric for methanol, but are strongly asymmetric for 4-fluorophenol, hence the differences in the O-H HB band shapes. The PE surfaces for the epoxide are effectively symmetric, but the PE for rotation about the HB has a single broad minimum for methanol, whereas with 4-fluorophenol there are two minima owing to attractive interactions between the phenyl group and the CH2 groups of the epoxide. The previously unknown β2H values for ethylene oxide and tetramethylethylene oxide are 0.36 and 0.58, respectively.Key words: asymmetric IR O-H bands, asymmetric potential energy surfaces, hydrogen-bonded complexes, hydrogen bond enthalpy, O-H frequency shift.

Theoretical Modeling of the N-H and N-D Stretching Bands of Hydrogen-Bonded 1-Methylthymine Crystal and Its Deuterated Form

2006 ◽  
Vol 2 (4) ◽  
pp. 205-219
Author(s):  
Marek Boczar ◽  
Łukasz Boda ◽  
Marek J. Wójcik
Keyword(s):  
Hydrogen Bonds ◽  
High Frequency ◽  
Molecular Crystals ◽  
Low Frequency ◽  
Unit Cell ◽  
Fermi Resonance ◽  
Bending Vibrations ◽  
Theoretical Simulation ◽  
Stretching Vibrations ◽  
Centrosymmetric Dimers

Theoretical model for vibrational interactions in the hydrogen bonds in molecular crystals with four molecules forming two centrosymmetric dimers in the unit cell is presented. The model takes into account anharmonic-type couplings between the high-frequency N-H(D) and the low-frequency N•••O stretching vibrations in each hydrogen bond, resonance interactions (Davydov coupling) between equivalent hydrogen bonds in each dimer, resonance interdimer interactions within an unit cell and Fermi resonance between the N-H(D) stretching fundamental and the first overtone of the N-H(D) in-plane bending vibrations. The vibrational Hamiltonian, selection rules, and expressions for the integral properties of an absorption spectrum are derived. The model is used for theoretical simulation of the νs stretching bands of 1-methylthymine and its ND derivative at 300 K. The effect of deuteration is successfully reproduced by our model.

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