Proportions of Species Observed in Jet Spectroscopy−Vibrational Energy Effects:  Histamine Tautomers and Conformers

Abstract

It has been found possible to understand the observed relative abundances of the several different tautomers and conformers observed in the millimeter-wave jet spectroscopy of histamine by using ab initio energy calculations at the MP2/6-311++G(d,p) level and including an estimate of thermal free energies. Conformational relaxation, previously found to be important for several molecules of comparable complexity, has been found to have a significant role in determining the relative abundances of histamine species detectable in the cooled jet, the present example being indicative that this phenomenon is expected to influence generally the interpretation of multiconformational jet spectra. The need to include free-energy corrections is also noteworthy. The thermal equilibration of histamine tautomers in the gas phase within the millimeter-wave spectrometer prior to jet expansion has been confirmed experimentally by comparing abundances measured with high surface area and low surface area inlet systems. Thus, it is valid to use relative abundances to indicate relative free energies. The presented ab initio calculations yield spectroscopic constants (e.g., planar moments) that tighten the identification of the four species detected in jet spectroscopy. In particular, the identity of the least abundant species (3 G-Ic) is confirmed. Nitrogen quadrupole coupling constants computed at the MP2/6-311++G(d,p) level are in noticeably better agreement with experiment than previously reported calculations, it being noteworthy that they require no scaling for the 14N nuclear quadrupole moment.

Mapping the conformational free energy of aspartic acid in the gas phase and in aqueous solution

“…The "effective population" of the observed conformers in the supersonic jet is a consequence of population due to their relative energies plus the possible contributions from relaxation of other conformers. 48,49 Using our nomenclature, the relative abundances estimated in experiments follow the ranking 1CaC1 > 1CbC1 ≈ 2TaC1 > 1CaT2 ≈ 2TbC1 > 1CbC3, with conformers attributed according to the MP2 calculations of the rotational constants. 15 Correlating relative free energies and abundances, this trend is in overall good agreement with the results obtained with metadynamics and the CHARMM26 force-field, with the exception of 1CbC3, which belongs to basin C and is thus lower in free energy than expected.…”

Collision-induced conformational changes in glycine

“…17,[32][33][34][35] It arises from the need to both ͑a͒ heat the sample of molecules to obtain a gas-phase concentration that is sufficient for spectroscopic measurement and ͑b͒ cool the molecules to obtain a clear spectrum. This is illustrated for a typical IR or microwave spectroscopy experiment in Fig.…”

“…17,[32][33][34][35] Conformers with large barriers to interconversion are expected to be trapped during cooling and maintain their original, high-temperature population. On the other hand, conformers exhibiting low-energy pathways leading to more stable structures are more likely to be missing from the final spectrum.…”

Cold collisions catalyse conformational conversion

“…2,3 In particular, structures can be observed that have high potential energies and the observed structural distributions are therefore frequently not in thermal equilibrium with the other degrees of freedom. [2][3][4][5][6][7][8][9][10] This can be rationalized by the height of the barriers that separate potential energy minima which might be hard to overcome under the conditions prevailing in the molecular beam experiments. The study of the potential energy landscape of gas-phase molecules and the development of experimental methods to manipulate the conformational distribution of these species currently is an active field of research.…”