Diatomic molecules spectroscopic constants

This table lists the leading spectroscopic constants and equilibrium internuclear distance in the ground electronic state for selected diatomic molecules. The constants are those describing the vibrational and rotational energy through the expressions ... 

Applications.—Diatomic Molecules. Spectroscopic methods provide much more precise information on diatomic molecules—the equilibrium bond length, re, and the harmonic and anharmonic potential constants— than electron diffraction measurements can at present supply. In addition, the theory interrelating the different types of bond-length parameter is very simple for diatomic molecules. Thus electron diffraction studies of diatomic molecules serve mainly as a test of electron diffraction technique. Accurate studies have been carried out for Nj, O2," NO, Clj, and I2." Typical results are shown in Table 3. 

Table 3 Comparison of electron diffraction and spectroscopic determinations of re for some diatomic molecules. Spectroscopic potential constants were used in the conversion of the electron diffraction r to re-Spectroscopic re values from reference 2d. For diatomic molecules (only) it is more convenient to compare re values. All distances in A...

Contains IR, mass, electronic/vibrational, and UV-VIS spectra as well as constants of diatomic molecules (spectroscopic data) and ion energetics data, etc., drawn from various evaluated sources. 

Force Constants for Bond Stretching Fundamental Vibrational Frequencies of Small Molecules Spectroscopic Constants of Diatomic Molecules Infrared Correlation Charts... 

More often than not, the following spectroscopic constants are available for a diatomic molecule ... 

Suchard, S. N., and Melzer, J. E., in Spectroscopic Constants for Selected Homonuclear Diatomic Molecules, Vol. II. Report SAMSO-TR-76-31,1976. 

Bourcier, S., Spectroscopic Data Relative to Diatomic Molecules, vol. 17 of Tables of Constants and Numerical Data, Pergamon, New York, 1970. A comprehensive collection of experimental data. 

It is generally true that quantum chemists would like to perform all calculations to the highest possible accuracy. However, this is often computationally feasible only for small systems, because of the rapid increase in computer time and perhaps storage requirements of the calculations with increasing size of the molecule. In many cases it is not even necessary to perform such calculations for the purposes of prediction or interpretation, since the accuracy to which results need be known is often inversely proportional to the size of the system. That is, for diatomic molecules it is generally necessary to compute spectroscopic constants or intensities very accurately, because these quantities can often be determined to high accuracy by experiment. For systems of a dozen or more atoms it is seldom necessary to have such accurate results, because the experimental accuracy is lower and the problems quantum chemists are called on to answer will consequently often involve lower accuracy. 

S. N. Suchard, "Spectroscopic Constants for Selected Heteronuclear Diatomic Molecules, Aerospace report No. TR-0074 (4641)-6, 1974. 

This Report deals with the calculation of spectroscopic constants both for diatomic and for polyatomic molecules, while concentrating on the former. The spectroscopic constants included are restricted to those measured in high-resolution gas-phase work. We cover the whole range of complexity in computation since this is determined not by the method used in computing the expectation value, but by the quality of the wavefunction used. Generally the wavefunctions used are of the ab initio type, but their quality will depend on the size and type of basis set employed as well as the method. 

By Use of Model Potentials. Most of the calculations performed on diatomic molecules have preferred the approach of fitting the calculated points to some model curve, and extracting the spectroscopic constants from the known relationships between them and the parameters of the model potentials. Most frequently used has been the polynomial in R. Generally an expression in positive powers of R is used,... 

The coefficients m,f x,f Be, a , De,. .. are the quantities determined from a spectroscopic analysis of a diatomic molecule, and our problem is to relate these to the force constants in the potential energy function,... 

Table 2 Observed spectroscopic constants and calculated potential constants for a selection of diatomic molecules 1... 

B. Rosen, Spectroscopic Data Relative to Diatomic Molecules , International Tables of Selected Constants, Pergamon Press, Oxford, 1971, Vol. 17. 

In practice, the result of the perturbation treatment may be expressed as a series of formulae for the spectroscopic constants, i.e. the coefficients in the transformed or effective hamiltonian, in terms of the parameters appearing in the original hamiltonian, i.e. the wavenumbers tor, the anharmonic force constants , the moments of inertia Ia, their derivatives eft , and the zeta constants These formulae are analogous to equations (23)—(27) for a diatomic molecule. They are too numerous and too complicated to quote all of them here, but the various spectroscopic constants are listed in Table 3, with their approximate relative orders of magnitude, an indication of which parameters occur in the formula for each spectroscopic constant, and a reference to an appropriate source for the perturbation theory formula for that constant. 

It is instructive to consider two of the formulae for the spectroscopic constants in more detail, and for this we choose — and xrs for an asymmetric top, these being respectively the coefficients of (vT + )J, the vibrational dependence of the rotational constant, and (vr + i)(fs + i), the vibrational anharmonic constant quadratic in the vibrational quantum numbers. As for diatomic molecules these two types of spectroscopic constant provide the most important source of information on cubic and quartic anharmonicity, respectively. The formulae obtained from the perturbation treatment for these two coefficients in the effective hamiltonian are as follows ... 

Spectroscopic Constants for Selected Heteronuclear Diatomic Molecules (Air Force Report No. SAMSO-TR-74-82), Vol. II, p. N102. 1982. 

NIST also maintains a website called the NIST Chemistry WebBook (http //webbook. nist.gov), which provides access to a broad array of data compiled under the Standard Reference Data Program. This site allows a search for thermochemical data for more than 7000 organic and small inorganic compounds, reaction thermochemistry data for over 8000 reactions, IR spectra for over 16,000 compounds, mass spectra for over 15,000 compounds, UV/VIS spectra for over 1600 compounds, electronic and vibrational spectra for over 5000 compounds, spectroscopic constants of over 600 diatomic molecules, ion energetics data for over 16,000 compounds, and thermophysical properties data for 74 selected fluids. The site allows general searches by formula, name, CAS registry number, author, and stracture and also a few specialized searches by properties like molar mass and vibrational energies. 

---