Infrared, Raman and Microwave Spectra

Infrared, Raman, and Microwave Spectra.—Brief details of studies which provide information on conformations and electron distributions are summarized at the end of this section. A comparison of the polarized Raman spectra for (C Hj)2S02 with far-i.r. data for Mc2S02 and polarized i.r. spectra for aryl thiocyanates and selenocyanates represent less routine studies. 

Infrared, Raman, and Microwave Spectra.—Interpretation of i.r. data in terms of hydrogen-bonding equilibria has been reported for thiols, - sulphides, and MeSOjNMea. A diminished H-bonding capacity is demonstrated for a sulphonamide compared with the corresponding carboxamide. ... 

The vapor-phase Raman spectrum of SF5CI (17), the argon-matrix Raman and infrared spectra of SF5CI and SFsBr (18), and the vapor-phase infrared and liquid-phase Raman spectra of SFsBr 19), as well as photoelectron diffraction 20) and microwave spectra of SF5CI 21) and SFjBr 22) have been reported. The ionization potential of SF5 (9.65 eV) has been measured by photoionization mass spectrometry of SF5CI (23). 

Since the potential-energy function for low-frequency vibrations involves weak force constants, the function is sensitive to intermolecular forces, which can reach comparable magnitudes to the intramolecular ones at short intermolecular distances. Thus in the liquid states the intramolecular levels are so seriously broadened as to make them difficult to observe, while in crystals the inversion barriers are drastically altered. Thus the most meaningful spectra are necessarily observed in the gas phase, and this delayed the development of the subject until suitable far infrared, laser Raman and microwave techniques were developed, as summarized below. 

It is possible to deduce the same kinds of structural information from Raman spectra as from infrared and microwave spectra. From the selection rule for rotation, Eq. (23.7-3a), the Raman shift of the Stokes rotational lines of a diatomic molecule is given in the rigid-rotor approximation by... 

Other studies, such as infrared and Raman spectra of gaseous benzene, neutron diffraction studies of crystalline benzene, and electron diffraction and microwave spectral studies, are equally incapable, according to critical analysis [87AG(E)782], of resolving unanimously the Dih—Deh structural dilemma of the benzene molecule. Furthermore, no decisive conclusion could be drawn from photoelectron spectra or H—NMR spectrum measurements of benzene molecules in a liquid crystal environment. The latter experiments merely indicate that the average lifetime of a Dih structure (if it appears on the PES) is less than 10 4 sec corresponding to the energy barrier of the Dih- >D6h-+D h interconversion of approximately 12 kcal/mol. 

Information about the structure of gas molecules haB been obtained by several methods. Spectroscopic studies in the infrared, visible, and ultraviolet regions have provided much information about the simplest molecules, especially diatomic molecules, and a few polyatomic molecules. Microwave spectroscopy and molecular-beam studies have yielded very accurate interatomic distances and other structural information about many molecules, including some of moderate complexity. Molecular properties determined by spectroscopic methods are given in the two books by G. Herzberg, Spectra of Diatomic Molecules, 1950. and Infrared and Raman Spectra, 1945, Van Nostrand Co., New York. The information obtained about molecules by microwave spectroscopy is summarised by C. H. Townes and A. L. Schawlow in their book Microwave Spectroscopy of Gases, McGraw-Hill Book Co., New York, 1955. 

Microwave, infrared, Raman, visible and UV spectra are all used extensively for identification. In the gas phase these show sharp lines so that identification is easy. In solution, the complexity of the spectra gives them sufficient features to make them recognizably specific to the molecule in question. 

The situation is not as clearcut for distances derived from rotational constants which can be determined by microwave (MW), high-resolution infrared (IR) and rotational Raman (Ra) or UV spectra. The r0 parameters are obtained from the rotational constant of the vibrational ground state. For polyatomic molecules these parameters have no direct... 

The molecule is pyramidal, having C3v symmetry with the nitrogen atom at the apex. The molecular dimensions have been determined by electron diffraction (266) and by microwave spectroscopy (161,271). The molecule with this symmetry will have four fundamental vibrations allowed, both in the infrared (IR) and the Raman spectra. The fundamental frequency assignments in the IR spectrum are 1031, vt 642, v2 (A ) 907, v3 (E) and 497 cm-1, v4 (E). The corresponding vibrations in the Raman spectrum appear at 1050, 667, 905, and 515 cm-1, respectively (8, 223, 293). The vacuum ultraviolet spectrum has also been studied (177). The 19F NMR spectrum of NF3 shows a triplet at 145 + 1 ppm relative to CC13F with JNF = 155 Hz (146, 216, 220,249, 280). 

Since the Raman effect involves two spin-one photons, the angular-momentum selection mle becomes A J = 0, 2. This gives rise to three distinct branches in the rotation-vibration spectra of diatomic and linear molecules the 0-branch (A / = —2), the Q-branch (A J = 0) and the S-branch (A J = - -2). All diatomic and linear molecules are Raman active. Raman spectroscopy can determine rotational and vibrational energy levels for homonuclear diatomic molecules, which have no infrared or microwave spectra. 

Vibrational frequencies are those assigned by Lide et al. (5) based on additional microwave and infrared data for the gaseous and matrix-isolated phases. Assignment of the three fundamentals near 544 cm" and the two near 388 cm" is supported by gas-phase Raman spectra (6), by combination and overtones in the infrared (6) and by force-field calculations (7). 

Further information on the force field comes from the observation of vibration/rotation interaction constants. These are obtained from infrared and Raman spectra at high resolution and from microwave spectra observed in vibrationally excited states. They are of the following types ... 

In practice, one usually defines a training set of molecules and associated experimental properties and fits the relevant data with an assumed force field.The next step is to test the results on molecules and data outside the training set. Experimental data that depend on the energy surface and that may be used to determine the parameters of the intramolecular interactions consist mainly of gas-phase structural data derived from microwave spectra or electron diffraction patterns, crystal structures derived from X-ray and inelastic neutron scattering measurements, and vibrational frequencies obtained from infrared and Raman spectra. Torsional barriers are derived from NMR band shapes and relaxation times, whereas conformational energies are determined from spectroscopic and thermochemical data. Nonbonded parameters are determined mainly from... 

An important advantage of Raman spectroscopy is that one does not have to work in the infrared and microwave regions of the spectrum. The region used is determined by the choice of frequency of the incident light and can be the visible region. Also, with Raman spectra it is substantially less difficult to work with aqueous systems. 

In modem papers ground state constants are frequently reported with cited uncertainties lxl0" cm" (3 kHz) from infrared work and 1 x lO cm" (0.3 MHz) from Raman studies. In band spectra, two sets of rotational constants are obtained, those of the upper and lower states involved in the transition, and a statistical treatment allows the differences between the constants to be determined to precisions approaching or eqnal to microwave uncertainties (1 kHz or less). Thus equilibrium rotational constants of polar molecules can be quite precisely calculated by using microwave-determined Bq constants and infrared-determined a constants. When the values of some of these a constants are missing, they can be substituted by reliable ab initio values. Despite the recent instmmental improvements, the resolution available from both infrared and Raman studies is still much lower than that from microwave spectroscopy, and therefore, studies are limited to fairly small and simple molecules. However, these techniques are not restricted to polar molecules as is the case for microwave spectroscopy, and thus... 

Most of the vibrational and rotational spectra obtained before the second world war were measured using Raman methods. Interest in Raman then declined as infrared and microwave absorption instrumentation developed, but the introduction of visible lasers in the early 1960s has led to dramatic renaissance in Raman spectroscopy. As well as decreasing the acquisition time and increasing the sensitivity of conventional Raman spectra by orders of magnitude, the high power and coherence properties of laser radiation has spawned a host of new nonlinear Raman spectroscopies, some of which can be performed without a... 

Infrared and Microwave Studies.—The i.r. and Raman spectra of acetonitrile-borane (CH3CN.BH3), in particular the compound containing the B isotope, have been recorded and analysed, and molecular parameters obtained from the microwave spectrum have been reported for pivalonitrile-borane (BuCN.BHg). Oxybis(divinylborane) has been synthesized by the limited hydrolysis of chlorodivinylborane, and its vibrational spectra have been analysed. ... 

The low melting point of hydrazine hydrate seen in Table 1.2 is consistent with a solid crystal structure that is somewhat different from that of anhydrous hydrazine (Figure 1.2) [4,5]. The low melting point of the hydrate indicates that its crystal is held together by relatively weak chemical forces like hydrogen bonds. Infrared, Raman, microwave. Nuclear Magnetic Resonance (NMR), photoelectron spectra, and X-ray diffraction have been used to elucidate the structure of the crystal and bonding in these molecules. Both hydrazine and hydrazine hydrate... 

It is well established that disulfur difluoride (S2F2) exists in two isomeric forms, the nonplanar disulfane FSSF and the branched thiosulfoxide form p2S=S, with the latter found to be the more stable isomer. Both isomers have been characterized by microwave spectroscopy, mass spectrometry, infrared and Raman spectroscopy as well as photoelectron spectra [6] (and refer-... 

An electric dipole operator, of importance in electronic (visible and uv) and in vibrational spectroscopy (infrared) has the same symmetry properties as Ta. Magnetic dipoles, of importance in rotational (microwave), nmr (radio frequency) and epr (microwave) spectroscopies, have an operator with symmetry properties of Ra. Raman (visible) spectra relate to polarizability and the operator has the same symmetry properties as terms such as x2, xy, etc. In the study of optically active species, that cause helical movement of charge density, the important symmetry property of a helix to note, is that it corresponds to simultaneous translation and rotation. Optically active molecules must therefore have a symmetry such that Ta and Ra (a = x, y, z) transform as the same i.r. It only occurs for molecules with an alternating or improper rotation axis, Sn. 

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