J. Parmentier

FT-IR spectrometric study of N-t-butoxycarbonylglycine N′,N′-dimethylamide and its interaction with proton donors

FT-IR spectra of N-t-butoxycarbonylglycine-N′,N′-dimethylamide and its deuteriated ND counterpart, in solution and in the solid state, have been analysed between 3800 and 400 cm–1. An assignment is proposed for some important vibrations, especially for the so-called amide vibrations, by comparison with literature data on similar molecules. The frequency shifts and intensity changes of the vibrations sensitive to the physical state of this model dipeptide strongly suggest that intermolecular hydrogen bonds involving mainly the NH and CO (amide) groups are formed in the solid state. Thermodynamic parameters (equilibrium constants and enthalpies) of the hydrogen-bond formation with phenol derivatives have been determined in carbon tetrachloride and in 1,2-dichloroethane. The complexes investigated are of medium strength. The FT-IR spectra in the νCO region and the double perturbation of the νNH band assigned to the five-membered intramolecular NH ⋯ OC hydrogen-bonded ring clearly show that complex formation occurs at both the amide and urethane carbonyl groups. About 45% of the complexes are formed on the CO (urethane) function, almost independent of the acidity of the phenols. The unusual broadness of the OH ⋯ OC complex band originates from the superposition of two complex bands. N-t-Butoxycarbonylglycine-N′,N′-dimethylamide can be considered as being built up from two model molecules, methyl-N-methylcarbamate and N,N-dimethylacetamide.

FT-IR study of N-tertiobutoxycarbonylproline-N′-methylamide self-association in carbon tetrachloride and influence of the solvent on the conformation

Abstract The self-association of N-tertiobutoxycarbonylproline-N′-methylamide (N-tBoc-Pro-NHMe) is investigated in carbon tetrachloride. The insertion constant of a second N-tBoc-Pro-NHMe molecule is equal to 31 mol−1. An intermolecular hydrogen bond is formed between the NH bond and the oxygen atom of the amide function. The wavenumber of the ν(NH) stretching vibration of the extended species is inversely correlated with the Onsager parameter of the solvent. Comparison with literature data shows that N-acetylproline-N′-methylamide (N-Ac-Pro-NHMe) has a greater tendency to form C7 conformers.

Infrared Study of the Interaction Between Carbamates and Proton Donors

Abstract Owing to its great importance as a structural link in proteins, the NH-CO amide group has been the subject of numerous experimental and theoretical investigations. In contrast, the -NH-COO-carbamate group has been re atively ignored despite its importance in biologically ac ive molecules such as local anaesthetics1–2. U p to now, no experimental studies concerning the proton acceptor ability of carbnmates have appeared in the literature. It must be pointed out however thnt ab initio calculations were carried out for the interaction between simple carbarnates and model molecules such as N- methylacetamide or phenol3. In this work, the complexes between N-methyl methyl-carbamate (MMC) and N,N-dimethyl methylcarbamate (DMMC) and model proton donors (phenol derivatives) are studied by IR or FT-IR spectroscopy

Theoretical and x-ray photoelectron spectrometric studies of the basicity of a glycine dipeptide

Abstract The proton acceptor ability of a glycine dipeptide RO(O)CNHCH 2 C(O)N(CH 3 ) 2 is investigated. Ab initio calculations are performed on N-methoxycarbonylglycine N′,N′-dimethylamide (R  CH 3 ) and compared with the experimental X-ray photoelectron spectrum of N- t -butoxycarbonylglycine N′,N′-dimethylamide (R  C(CH 3 ) 3 ). The HF/6-31G* optimized structure shows that the dipeptide is stabilized by an intramolecular hydrogen bond between the NH and amide carbonyl groups. The molecular electrostatic potential and the atomic changes fitted to this potential are calculated. The HF/6-31G* 1s orbital energies of the carbon and oxygen atoms are computed and compared with the experimental X-ray photoelectron spectra. The difference between the proton affinities (PA) of the oxygen atoms can be extracted from these data. The PA of the urethane carbonyl oxygen is 26–29 kJ mol −1 higher than that of the amide carbonyl oxygen and 85–90 kJ mol −1 higher than that of the alcoxy oxygen. These results are compared with infrared data, which have shown that normal hydrogen bond formation with hydroxylic proton donors occurs at the oxygen atoms of the two carbonyl groups but that the urethane oxygen is the only protonation site.

Infrared study of the proton acceptor ability of 1-pyrrolidine-carboxaldehyde

Abstract The hydrogen bond interaction between 1-pyrrolidinecarboxaldehyde (PCA) and phenols (pKa = 10.20 - 7.75) or trifluoroacetic acid (TFA) is investigated by infrared spectrometry. The formation constants between PCA and phenols determined in carbontetrachloride range between 80 and 2050 1mol−1 (298 K) and the enthalpies of complex formation between 25 and 33 kJmol−1. The hydrogen bond is formed on the oxygen atom of the carbonyl function but with TFA, a small proportion of the complexes are formed on the nitrogen atom of the pyrrolidine ring. The infrared spectra of the binary PCA-TFA mixtures suggest that the OH …O hydrogen bond is strong and asymmetrical.

FT-IR Study of the Interaction Between 1,3-Dimethyluracil and the Lithium Ion

Abstract Protonation and complexation of nucleic acid bases with metal ions have a profound influence on their structure and reactivity.1–3 Although the interaction between carbony bases and the Li+ ion has been studied by vibrational spectroscopy and ab initio calculations8–15, no experimental results are available for the complexation between this ion and carbonyl bases derived from nucleic acids. In this work, the interaction between 1,3-dimethyluracil (DMU)