Combining density functional theory (DFT) and collision cross-section (CCS) calculations to analyze the gas-phase behaviour of small molecules and their protonation site isomers

Electrospray ion mobility-mass spectrometry (IM-MS) data show that for some small molecules, two (or even more) ions with identical sum formula and mass, but distinct drift times are observed. In spite of showing their own unique and characteristic fragmentation spectra in MS/MS, no configurational or constitutional isomers are found to be present in solution. Instead the observation and separation of such ions appears to be inherent to their gas-phase behaviour during ion mobility experiments. The origin of multiple drift times is thought to be the result of protonation site isomers ('protomers'). Although some important properties of protomers have been highlighted by other studies, correlating the experimental collision cross-sections (CCSs) with calculated values has proven to be a major difficulty. As a model, this study uses the pharmaceutical compound melphalan and a number of related molecules with alternative (gas-phase) protonation sites. Our study combines density functional theory (DFT) calculations with modified MobCal methods (e.g. nitrogen-based Trajectory Method algorithm) for the calculation of theoretical CCS values. Calculated structures can be linked to experimentally observed signals, and a strong correlation is found between the difference of the calculated dipole moments of the protomer pairs and their experimental CCS separation

Infrared diode laser jet spectroscopy of the van der Waals complex (N2O)(2)

The rotationally resolved spectrum of the dimer (N2O)2 has been recorded in the region of the monomer N2O v3 vibrational band using a diode laser absorption spectrometer which incorporates a multiple-pass cell and a pulsed-jet expansion. A double-modulation technique involving a fast modulation of the laser frequency and a slow modulation of the gas pulse has been developed in the experiments. The spectrum is completely analysed and the rotational constants and effective structures are accurately determined for both the ground and the excited vibrational states. The centrosymmetric slipped parallel structure of (N2O)2 is well explained by an intermolecular potential containing a combination of electrostatic interactions between distributed multipoles of different monomers and atom-atom Lennard-Jones potentials to describe the repulsion and dispersion interactions

High-Resolution Infrared Diode Laser Spectroscopy of Ne-N2O, Kr-N2O, and Xe-N2O.

The rotationally resolved spectra of the van der Waals complexes Ne-N2O, Kr-N2O, and Xe-N2O have been investigated in the region of the nu3 N2O monomer vibrational band using a diode laser absorption spectrometer that is incorporated with a multipass cell and a pulsed jet. The spectra of these three complexes are completely analyzed using a normal asymmetric rotor Hamiltonian, and the effective molecular constants are accurately determined for both the ground and the excited vibrational states. These results show that, like Ar-N2O, the complexes have a T-shaped configuration in which the rare gas atom prefers to lie near to the oxygen side of N2O. The band origins of Rg-N2O (Rg = Ne, Ar, Kr, and Xe) are observed to shift by 0.36125, 0.15038, -0.10131, and -0.49066 cm-1 from that of the monomer, respectively. These band origin shifts are well explained by a simple model for the intermolecular potential. Copyright 1998 Academic Press