Imidazole Trimer-Water Complexes in Superfluid Helium Nanodroplets: Water Stretching Modes

Abstract

Imidazole (IM) is an important moiety, having a proton donor and acceptor, in nucleic acid bases, i.e., adenine and guanine, and plays an important role in the intermolecular interactions of biological processes so that many theoretical and experimental studies of IM and its clusters have been investigated. Previously, we have presented and characterized the high-resolution infrared laser spectra of IM, imidazole dimer (IMD) and two isomers of IMand IMDwater complexes (IMW and IMDW) formed in helium nanodroplets. Recently, we have extended the study of imidazole clusters to imidazole trimer (IMT) reporting the vibrational spectra of IMT in the free NH spectral region and found out that the IM molecules tend to form linear structures due to their dipole-dipole interactions in helium nanodroplets. The unambiguous free NH band assignment was made by the experimental techniques, such as pick-up oven temperature dependence and vibrational transition moment angle (VTMA), and density functional theory (DFT) calculations. In the present note, higher order complexes, IMT with one water molecule (IMTWs), formed in He nanodroplets are presented. To the best of our knowledge, there is no report on the spectroscopic studies of IMTWs systems due to experimental difficulties in obtaining well resolved spectra. In this study we have used advantage of using helium nanodroplets since the very weak interactions between the target molecules and the helium give rise to almost unperturbed environments from that of the gas phase. Furthermore, the temperature of the helium nanodroplets is extremely low, 0.37 K, thus the spectral broadening is significantly reduced making the He droplets as an ideal matrix for infrared spectroscopy. Using the helium nanodroplet techniques for obtaining well resolved spectra, we were able to measure the angle between the transition and permanent dipole moments of the target molecules, called the vibrational transition moment angles (VTMAs), which turned out to be a very sensitive structural and unambiguous vibrational band assignment tool. Furthermore, VTMAs can be also easily calculated and compared with the experimental values obtained in this study. The use of VTMAs for the specific tautomer and vibrational band assignments of small biomolecules is well described in our previous reports. In this note we present the first spectroscopic investigation on the IMTW systems applying the VTMA techniques in He nanodroplets. Figure 1 shows the two lowest energy structures for IMTWs (> NH...OH2 and > N: ...H-O-H), where IMT is acting as a proton acceptor and donor from the nitrogen atom of the IM ring in the two lowest energy isomers, IMTW1 and 2, respectively. The structures were optimized at the B3LYP/6-311++G(d,p) level of theory and the calculated vectors representing the directions of the permanent electric dipole moments (solid arrows) and the vibrational transition moments (empty arrows) are superimposed onto the clusters. The magnitudes of the various moments for IMTWs are given in Table 1. It is clear from the figure that the patterns of VTMAs for the two isomers are quite different, making them a useful tool for distinguishing these