Molecular Structure
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Molecular structure (molecular geometry) is a one of the most fundamental properties of molecules, which characterizes spatial positions of the atoms that constitute a molecule. This term is usually referred to the geometrical structure of molecules in gas phase.
Types of molecular structure
Abbreviations are:
- QC — Quantum-chemical calculations
- GED — Gas electron diffraction
- MW — Microwave spectroscopy
- HRMS — High resolution molecular spectroscopy
| Type | Description | Methods |
|---|---|---|
| re | Equilibrium structure. Appears as a result of Born-Oppenheimer approximation in quantum mechanics. | QC |
| rg | Thermally averaged structure.[gt 1] rg distances are centers of gravity for corresponding distribution functions P(r). | GED |
| ra | Effective average structure parameter type[gt 1] resulting from fitting the theoretical model equation: Mtheo.(s)=const. Σi≠jN gij(s) exp(-0.5 lij2s2) sin[s(rij - κijs2)]/rij to experimental scattering data Mexp.(s) with respect to the parameters rij, lij and κij (distances, amplitudes and anharmonicity or "asymmetry" parameters). The resulting structure parameters rij are interpreted as "average structure parameters" of type "ra". gij(s) is the interatomic scattering cross sections function (for which ab-initio calculation based tabulated values are usually used). N is the number of scattering centers ("atoms"). In general it is not possible to assign a set of atoms to positions in a Euklidian space R3 such that a complete set of ra parameters correspond to the interatomic distances of the atom positions in R3 space. This is known also as the "shrinkage" problem, since due to zero point vibrational motions already for a single CO2 in the vibrational ground state: 2 ra(C-O) > ra(O-O'). | GED |
| rα = rh0 | Distances between average nuclear positions.[gt 2] | GED, MW, HRMS |
| r0α, rz | Distances between average nuclear positions at 0 K, i.e. for ground vibrational state.[gt 2] | GED, MW, HRMS |
| rh1 | Distances between average nuclear positions calculated taking into account nonlinear relationships between internal and Cartesian coordinates.[gt 2] | GED, MW, HRMS |
| ra3,1 | Semi-experimental equilibrium structure obtained using corrections calculated by Shrink program utilizing cubic force fields. | GED, MW, HRMS |
| rsee | Semi-experimental equilibrium structure obtained using a set of theoretically calculated corrections to experimental and/or operational data. | GED, MW, HRMS |
| rs | Effective structure derived from isotopic differences in rotational constants. | MW, HRMS |
| r0 | Effective structure derived from rotational constants of zero-point vibrational level. | MW, HRMS |
| rm | Effective structure derived from the mass-dependence method of Watson. | MW, HRMS |
| rρm | Effective structure similar to rm obtained by a slightly modified method of Harmony et al. | MW, HRMS |
- ↑ 1.0 1.1 Interatomic distances are averaged over molecular vibrations for a given temperature. Generally such distances are geometrically inconsistent.
- ↑ 2.0 2.1 2.2 In contrast to rg, where subject to averaging are interatomic distances, here atomic positions are averaged. Therefore this structure is geometrically consistent.
In some of the old GED papers the parameter types rg(0) and rg(1) are used. They are equivalent to normal rg and ra types, respectively.<bib id="Kuchitsu1959" />