Infrared spectrum of the complex of formaldehyde with carbon dioxide in argon and nitrogen matrices

Spectroscopic evidence of n → π* interactions involving carbonyl groups

n → π* has emerged as an important noncovalent interaction that can affect the conformations of both small- and macromolecules including peptides and proteins. Carbonyl-carbonyl (COCO) n → π* interactions involving CO groups are well studied. Recent studies have shown that the COCO n → π* interactions are the most abundant secondary interactions in proteins with a frequency of 33 interactions per 100 residues and, among the various secondary interactions, n → π* interactions are expected to provide the highest enthalpic contributions to the conformational stability of globular proteins. However, n → π* interactions are relatively weak and provide an average stabilization of about 0.25 kcal mol-1 per interaction in proteins. The strongest n → π* interaction could be as strong as a moderate hydrogen bond. Therefore, it is challenging to detect and quantify these weak interactions, especially in solution in the presence of perturbation from other intermolecular interactions. Accordingly, spectroscopic investigations that can provide direct evidence of n → π* interaction are limited, and the majority of papers published in this area have relied on X-ray crystallography and/or theoretical calculations to establish the presence of this interaction. The aim of this perspective is to discuss the studies where a spectroscopic signature in support of n → π* interaction was observed. As the "n → π* interaction" is a relatively new terminology, there remains the possibility of there being earlier studies where spectroscopic evidence for n → π* interactions was obtained but it was not discussed in light of the n → π* terminology. We noticed several such studies and, as can be expected, these studies were often overlooked in the discussion of n → π* interactions in the recent literature. In this perspective, we have also discussed these studies and provided computational support for the presence of n → π* interaction.

Brute Force Orientation of Matrix-Isolated Molecules: Reversible Reorientation of Formaldehyde in an Argon Matrix toward Perfect Alignment

Brute force orientation by an electric field is a promising way of controlling the orientation of polar molecules in the gas phase, but its application to condensed-phase molecules has been very limited. We studied the reorientation of formaldehyde molecules in a solid Ar matrix under the influence of a strong electric field using reflection absorption infrared spectroscopy. Asymptotically perfect alignment of the formaldehyde molecules along the field was achieved at field strengths exceeding 1×10⁸  V m^-1 . The vibrational bands of the aligned molecules exhibited a unidirectional Stark shift proportional to the field strength. The reorientation of the molecules was reversible despite the cryogenic solid environment of the system.

Laboratory studies of the newly discovered infrared band at 4705.2 cm-1 (2.1253 micrometers) in the spectrum of Io: the tentative identification of CO2

We discuss over 120 laboratory experiments pertaining to the identification of the new absorption band discovered by Trafton et al. (1991) at 4705.2 cm-1 (2.1253 micrometers) in the spectrum of Io. It is shown that this band is not due to overtones or combinations of the fundamental bands associated with the molecules (or their chemical complexes) already identified on Io, namely, SO2, H2S, and H2O. Thus, this band is due to a new, previously unidentified, component of Io. Experiments also demonstrate that the band is not due to molecular H2 frozen in SO2 frosts. Since the frequency of this band is very close to the first overtone of the nu 3 asymmetric stretching mode of CO2, we have investigated the spectral behavior of CO2 under a variety of conditions appropriate for Io. The profile of the Io band is not consistent with the rotational envelope expected for single, freely rotating, gaseous CO2 under Io-like conditions. It was found that pure, solid CO2 and CO2 intimately mixed in a matrix of solid SO2 and H2S produce bands with similar widths (5-10 cm-1), but that these bands consistently fall at frequencies about 10-20 cm-1 (approximately 0.007 micrometer) lower than the Io band. CO2 in SO2 : H2S ices also produces several additional bands that are not in the Io spectra. The spectral fit improves, however, as the CO2 concentration in SO2 increases, suggesting that CO2-CO2 interactions might be involved. A series of Ar : CO2 and Kr : CO2 matrix isolation experiments, as well as laboratory work done elsewhere, show that CO2 clustering shifts the band position to higher frequencies and provides a better fit to the Io band. Various laboratory experiments have shown that gaseous CO2 molecules have a propensity to cluster between 80 and 100 K, temperatures similar to those found on the colder regions of Io. We thus tentatively identify the newly discovered Io band at 4705.2 cm-1 (2.1253 micrometers) with CO2 multimers or "clusters" on Io. Whether these clusters are buried within an SO2 frost, reside on the surface, or are in a residual, steady-state "atmospheric aerosol" population over local coldtraps is not entirely clear, although we presently favor the latter possibility. The size of these clusters is not well defined, but evidence suggests groups of more than four molecules are required. The absorption strength of the 2 nu 3 CO2 cluster overtone determined in the laboratory, in conjunction with the observed strength of the Io band, suggests that the disk-integrated abundance of CO2 is less than 1% that of the SO2. Studies of the sublimation behavior of CO2 indicate that it probably resides predominantly in the cooler areas (< 100 K) of Io. The relative constancy of the Io feature over a variety of orbital phases suggests that the polar regions may contain much of the material. Some consequences of the physical properties of CO2 under conditions pertinent to Io are discussed. The presence of CO2 clusters on Io could be verified by the detection of any one of several other infrared bands associated with the CO2 molecule, of which the strongest are the nu 3 12CO2 asymmetric stretch fundamental near 2350 cm-1 (4.25 micrometers) and the nu 2 bending mode fundamental near 660 cm-1 (15.1 micrometers). Weaker bands that may also be detectable include the nu 3 13CO2 asymmetric stretch fundamental near 2280 cm-1 (4.39 micrometers), the 2 nu 2 + nu 3 combination/overtone band near 3600 cm-1 (2.78 micrometers), and the nu 1 + nu 3 combination band near 3705 cm-1 (2.70 micrometers).