Molecular Structure
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 | Average structure [gt 1] used for calculation of molecular scattering intensity in GED theory. The parameters correspond to rij from the model equation M(s)=const. Σi≠jN gij(s) exp(-0.5 lij2) sin[s(rij - κijs2)]/rij with parameters lij interpreted as amplitudes of vibration, κij as anharmonicity or "asymmetry" parameters and interatomic scattering cross sections gij(s). N is the number of scattering centers ("atoms"). | 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" />