Perturbation of Pyridinium CIVP Spectra by N₂ and H₂ Tags: An Experimental and BOMD Study

Abstract

In cryogenic ion vibrational predissociation (CIVP) spectroscopy, the influence of the tag on the spectrum is an important consideration. Whereas for small ions there have been several studies showing that the tag effects can be significant, these effects are less understood for large ions, or for large numbers of tags. Nevertheless, it is commonly assumed that if the investigated molecular ion is large enough, the perturbations arising from the tag are small, and can therefore be neglected in the interpretation. In addition, it is generally assumed that the more weakly bound the tag is, the less it perturbs the CIVP spectrum. Under these assumptions, CIVP spectra are claimed to be effectively IR absorption spectra of the free molecular ion. Having observed unexpected splittings in otherwise unproblematic CIVP spectra of some tagged ions, we report Born-Oppenheimer molecular dynamics (BOMD) simulations that strongly indicate that mobility among the more weakly bound tags leads to the surprise splittings. We compared the behavior of two tags commonly used in CIVP spectroscopy (H2 and N2) with a large pyridinium cation. Our experimental results surprisingly show that the more weakly bound tag can, under the appropriate circumstances, perturb the CIVP spectra more than the more strongly bound tag, by not just shifting, but also splitting the observed bands. The more weakly bound tag had significant residence times at several spectroscopically distinct sites on the molecular ion. This indicates that the weakly bound tag is likely to sample several binding sites in the experiment, some of which involve interaction with the reporter chromophore.

Application of continuous wave quantum cascade laser in combination with CIVP spectroscopy for investigation of large organic and organometallic ions

“…Nitrogen was chosen as a buffer gas to exclude the potential presence of several tagomers in the ICR cell and perturbation of the spectra by more highly mobile tag molecules such as H 2 or D 2 . 31 Indeed, a spectrum of 1 features two bands: more intense at 2230 cm −1 and minor at 2240 cm −1 (Figure 1a) that could be assigned to the C≡C asymmetric (v as ) and symmetric (v s ) stretching in excellent agreement with the computed spectrum (Figure 1c). By comparison, the two bands are not separately resolved in the solidstate FT-IR of the corresponding neutral pyridine molecule, which shows only one slightly redshifted peak at 2224 cm −1 with a significantly larger FWHM (Figure 1b) due to the overlapping of the modes.…”

“…This method's characteristic feature is the employment of weakly-bound messenger species (tags) for measurement, which lowers the dissociation barrier, leaving only one available channel-the loss of non-covalently bound gas molecule. Although the tag effect on the resulted spectrum is a subject of discussion, 31,[50][51][52][53][54][55][56][57][58][59]60 from the point of view of tunable laser applications, the main advantage of the one-photon dissociation process is the use of much lower laser power compared to the one required by the IRMPD process. Typically, lasers based on nonlinear mixing, either inside or outside a resonator cavity [61][62][63] (also called optical parametric wavelength conversion) are used.…”

Increasing Complexity in a Conformer Space Step-by-Step: Weighing London Dispersion against Cation−π Interactions

“…Previously, we have executed computational studies, using Born–Oppenheimer molecular dynamics (BOMD), on the isobaric pyridinium cations tagged with either N 2 or H 2 . While there are some differences in binding energies among the different possible binding sites in the pyridinium cations, these differences are small, especially for the H 2 tag, where the absolute binding energies never exceed 2 kcal mol –1 and site differences appear to be very small.…”

“…Previously, we have executed computational studies, using Born−Oppenheimer molecular dynamics (BOMD), on the isobaric pyridinium cations tagged with either N 2 or H 2 . 48 While there are some differences in binding energies among the different possible binding sites in the pyridinium cations, these differences are small, especially for the H 2 tag, where the absolute binding energies never exceed 2 kcal mol −1 and site differences appear to be very small. For the N 2 tag, the binding is slightly stronger, with aryl or tert-butyl binding sites showing binding energies around 3 kcal mol −1 , and the positively charged N−H binding site, when accessible, showing a binding energy of 4 kcal mol −1 .…”

Comment on “Microhydration of Biomolecules: Revealing the Native Structures by Cold Ion IR Spectroscopy”

“…Both schemes are indirect action spectroscopies that require careful considerations when analyzing the resulting spectra. For the one-photon IRPD experiment, while the effects of the D 2 presence are usually quite small, its interaction with specific N–H and O–H groups can nonetheless cause a ∼10–50 cm –1 redshift in their infrared vibrational bands. ,, The H 2 O binding energy in GlyH + (H 2 O) is calculated (cam-B3LYP/def2TZVP) to be around 6500 cm –1 (77.8 kJ/mol), and therefore, the IRMPD scheme requires the absorption of 2–3 photons in the 2900–3800 cm –1 range studied by Saparbaev et al . The potential changes in absorption cross section after the absorption of the first photons, or variation in the rate of vibrational energy redistribution, may cause the intensity of features in IRPD and IRMPD spectra to differ. , The multiphoton nature of the IRMPD process can also make it more sensitive toward the higher-energy kinetically trapped conformers that require fewer photons for dissociation.…”