Infrared spectroscopy electronic structure calculations

Atkins CG, Banu L, Rowsell M, Blagojevic V, Bohme DK, Pridgen TD (2009) Structure of [Pb(Gly-H)](-i-) and the monosolvated water and methanol solvated species by infrared multiple-photon dissociation spectroscopy, energy-resolved collision-induced dissociation, and electronic structure calculations. J Phys Chem B 113 14457-14464... 

A combination of in situ infrared (IR) measurements and electronic structure calculations has been used to identify a number of intermediates in the stereospecific intramolecular arylation of lithiated D-benzyl carbamates." Coordination of the starting carbamate with the base generates a prelithiated complex, detectable by IR spectroscopy... 

Bis(imino)pyridine iron complex 5 as a highly efficient catalyst for a hydrogenation reaction was synthesized by Chirik and coworkers in 2004 [27]. Complex 5 looks like a Fe(0) complex, but detailed investigations into the electronic structure of 5 by metrical data, Mossbauer parameters, infrared and NMR spectroscopy, and DFT calculations established the Fe(ll) complex described as 5 in Fig. 2 to be the higher populated species [28]. 

O-H bond. Among such properties a prominent one is the ultraviolet absorption spectrum and the theory may therefore be used for the examination of some of the spectroscopic shifts which accompany the lactam-lactim tautomerization. Much caution must, however, be exercised in this respect. Thus, in a recent paper Kwiatkowski135,137 performed Pariser-Parr-Pople-type calculations on the electronic structure of hydroxypurines, essentially to interpret their ultraviolet spectra. In these calculations he assumed that these compounds exist predominantly in their lactim form, and the results of his calculations, at least for 6- and 8-hydroxypurine, did not seem to contradict this assumption. It is only in the case of the 2-hydroxy isomer that a particularly striking disagreement between theory and experiment led him to admit that this last compound may exist in the lactam form. Calculations carried out for this form gave, in fact, a more satisfactory agreement with experiment.138 As we have seen, unambiguous infrared spectroscopy evidence clearly show s that all three isomers exist essentially in the lactam form. This shows that ultraviolet absorption may provide only very uncertain evidence about the lactam-lactim tautomerism in hydroxypurines and related compounds. 

By combining high-level ab initio calculations with high-resolution infrared spectroscopy, the equilibrium bond lengths in x-frans-butadiene have been determined to an unprecedented precision of 0.1 pm. The values found for the pair of n-electron delocalized double bonds and the delocalized central single bond are 133.8 and 135.4 pm, respectively. The data provide definitive structural evidence that validates the fundamental concepts of n-electron delocalization, conjugation, and bond alternation in organic chemistry. 

CIRDLS = infrared diode laser spectroscopy ES = electronic spectroscopy, spectra with resolved vibrational structure MW = microwave spectroscopy force field calculations denote harmonic frequencies obtained on the basis of combined analysis of electron diffraction and vibrational spectroscopy data. 

About 25 years ago experimental and theoretical studies of molecular structures and conformational properties were done quite separately. Because theoretical methods have also become applicable for reasonably sized molecules, experimental investigators started to take advantage of these methods and included molecular mechanics (MM), semi-empirical, and later ab initio and/or density flmctional (DFT) calculations in their experimental analyses. Today, because computer programs are very easy to use and sufficient computer capacity is generally available, most experimental studies of gas phase structures by gas electron diffraction (GED), microwave (MW), or high-resolution infrared spectroscopy are combined with theoretical calculations. 

As mentioned in the previous papers, spectroscopy (sp) and electron diffraction (ed) have their own merits and demerits and, in many cases, a combined use of them should result in the most accurate structure for free molecules. The connections among the structural parameters determined by the ed and sp methods are shown in Table 1. Information about geometry comes from the bonded and non-bonded internuclear distances determined by ED and also from the rotational constants determined by sp. One has to test in the first place whether these values are consistent with each other. One of the experimental examinations is shown in Figure 1.t For BF3, the discrepancies between the measured ed intensities (dots) and those calculated from the rotational constants B0 (measured by Ginn etaL by high-resolution infrared spectroscopy), with corrections for vibrational effects, are well within the estimated limits of experimental error. The rz (B-F) distances determined by ed and sp independently are 1311 0.0012 A and 13113 ... 

The energies calculated from equation 6.1 for the infrared and Raman frequencies lie in the range of the vibrational and rotational motions of molecules. Infrared and Raman spectroscopy provide structural information once the various absorption bands are assigned to specific molecular vibrations. Similarly, the energies associated with the visible and ultraviolet radiation lie in the range of electronic transitions within the atoms and provide information about chemical bonding. Electromagnetic radiation is the most widely used because of the availability of sources and detectors and the interpretation of the data in... 

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