Infrared spectroscopy vibrational frequencies, calculation

The vibrational states of a molecule are observed experimentally via infrared and Raman spectroscopy. These techniques can help to determine molecular structure and environment. In order to gain such useful information, it is necessary to determine what vibrational motion corresponds to each peak in the spectrum. This assignment can be quite difficult due to the large number of closely spaced peaks possible even in fairly simple molecules. In order to aid in this assignment, many workers use computer simulations to calculate the vibrational frequencies of molecules. This chapter presents a brief description of the various computational techniques available. 

Unstable conformers of trans- and cis-hexatriene have been detected by means of the combination of matrix-isolation infrared spectroscopy and photoexcitation (or the high-temperature nozzle technique)84. Ab initio MO calculations at the HF/6-31G level have been performed for several conformers of 1,3,5-hexatriene93. The observed infrared bands of unstable conformers have been attributed to the gTt (major species) and gTg (minor species) conformers of /raw.s -hexalricne and the gCt conformer of cw-hexatriene93. It is noted that, in the previous paper93, the notation c is used for twisted structures for the sake of simplicity. The calculated torsional angles around C—C bonds for the gTt, gTg and gCt conformers are in the range between 32° and 45°. The observed and calculated vibrational frequencies of gTt and gCt are reported in Reference 93. 

Studies by Teplyakov et al. provided the experimental evidence for the formation of the Diels-Alder reaction product at the Si(100)-2 x 1 surface [239,240]. A combination of surface-sensitive techniques was applied to make the assignment, including surface infrared (vibrational) spectroscopy, thermal desorption studies, and synchrotron-based X-ray absorption spectroscopy. Vibrational spectroscopy in particular provides a molecular fingerprint and is useful in identifying bonding and structure in the adsorbed molecules. An analysis of the vibrational spectra of adsorbed butadiene on Si(100)-2 x 1 in which several isotopic forms of butadiene (i.e., some of the H atoms were substituted with D atoms) were compared showed that the majority of butadiene molecules formed the Diels-Alder reaction product at the surface. Very good agreement was also found between the experimental vibrational spectra obtained by Teplyakov et al. [239,240] and frequencies calculated for the Diels-Alder surface adduct by Konecny and Doren [237,238]. 

Infrared and Raman are also rapid spectroscopic techniques that have been useful in the characterization of electrophiles in the condensed phase. Many superelectrophiles are expected to possess characteristic or new vibrational modes. The harmonic vibrational frequencies and infrared intensities for the nitronium ion (N02+) and protonitronium ion (HNO22"1") have been estimated using ab initio molecular orbital calculations (Table 5).37 Although the vibrational modes for the superelectrophile (HN022+) clearly differ from that of the monocation, data were so far not reported for the superelectrophile using infrared and Raman spectroscopy. When nitronium salts were dissolved in excess HF-SbFs, no apparent... 

An impressive application of infrared and Raman spectroscopy was demonstrated in studies of superelectrophilic diprotonated thiourea, [H3NGSH jNfE 2+ 2AsFf,. The Raman spectrum (taken at — 110°C) corresponded reasonably well with calculated vibrational bands predicted by density functional theoiy.38 Coupled with computational methods for predicting vibrational frequencies, it is expected that vibrational spectroscopic techniques will be useful for the observations of these and other superelectrophiles. 

The most basic information that is needed for constructing a global potential energy surface for gas phase MD simulations is the structures and vibrational frequencies. The earliest information about gas-phase RDX molecular structures was obtained from theoretical calculations [54-58]. In 1984 Karpowicz and Brill [59] reported Fourier transform infrared spectra for vapor-phase (and for the a - and p -phase) RDX in 1984, however, their data precluded a complete description of the molecular conformations and vibrational spectroscopy. More recently, Shishkov et al. [60] presented a more complete description based on electron-scattering data and molecular modeling. They concluded that the data were best reproduced by RDX in the chair conformation with all the nitro groups in axial positions. 

An important task for theory in the quest for experimental verification of N4 is to provide spectral characteristics that allow its detection. The early computational studies focused on the use of infrared (IR) spectroscopy for the detection process. Unfortunately, due to the high symmetry of N4(7)/) (1), the IR spectrum has only one line of weak intensity [37], Still, this single transition could be used for detection pending that isotopic labeling is employed. Lee and Martin has recently published a very accurate quartic force field of 1, which has allowed the prediction of both absolute frequencies and isotopic shifts that can directly be used for assignment of experimental spectra (see Table 1.) [16]. The force field was computed at the CCSD(T)/cc-pVQZ level with additional corrections for core-correlation effects. The IR-spectrum of N4(T>2 ) (3) consists of two lines, which both have very low intensities [37], To our knowledge, high level calculations of the vibrational frequencies have so far only been performed... 

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. 

Several attempts have been made to determine the symmetry (and hence the conformation of the phosphazene ring) of halogenocyclo-phosphazenes in the solid, liquid, and solution states using infrared and Raman spectroscopy (2, 136, 249, 255, 255a, 422). With some exceptions, there is reasonable agreement between the structures determined by diffraction methods and those predicted by vibrational spectroscopy. The calculation of force constants in N3P3C16 and assignment of vibration frequencies have been discussed (118). 

In vibrational studies (IR/Raman, see Infrared Spectroscopy and Raman Spectroscopy), characteristic frequencies for Au bonds have been compiled, which are useful for suggestions regarding molecular synunetries and structures and to calculate force constants for the Au bonds. The values have been tabulated in handbooks, and they show consistent results for the individual groups of compounds. Moreover,... 

The molecular structure and bond lengths and angle were determined using microwave spectroscopy by Tyler and Sheridan (3 ). The vibrational frequencies were reported by Aynsley et. al. ( ) from the infrared spectrum, except for the bending frequency which is estimated from the values for CICN, BrCN and ICN, by comparison of bending force constants. The reasonable limits for this value as calculated from generous limits on the bending force constant are 405-450 cm... 

The radical cation formed upon ionization of ANI has been studied by different spectrometric techniques, including photoelectron, two-color photoionization, ZEKE70,80,199,212-226 and mass107,227-232 spectrometries. In most cases, the technique used has been coupled with infrared spectroscopy, which allowed the fine vibrational spectrum of the ion to be determined, in both line position and intensity. For example, the ZEKE photoelectron spectrum216 was recorded by exciting to the neutral S ( 52) excited state, and well-resolved vibrational bands of the cation were observed. In conjunction with quantum chemical calculations of fundamental frequencies, an assignment of the observed vibrational bands can thus be made. A few theoretical studies56,107,218,233,234 have also been devoted to the radical cation. 

Many of the recent studies that examine Raman and infrared spectroscopy have been mentioned in previous sections of this chapter.However, a vibrational spectroscopic smdy by Comerlato and coworkers used HE and B3LYP with the SBKJC EPC for tin to examine IR and Raman spectra of the anionic [NEt4]2[Sn(dmit)3] complex. Comparison of the calculated scaled frequencies to experimental values revealed that the B3LYP method is more accurate than the HE method. The latter method is well known to overestimate frequencies by about 10%. 

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