Bond lengths, angles, or dihedrals given molecule at different levels of theory
The next several pages ask for
first - a molceule
second - a bond length, angle or dihedral, by choosing 2, 3 or 4 atoms in the molecule
The atom numbering does not line up between different
methods for some species in the CCCBDB.
This can lead to some erroneous geometry comparisons.
We are working on correcting this.
Some experimental bond lengths are re values (bottom of well)
and some are r0 (vibrational zero point level).
The calculations yield re values.
For diatomics the experimental values are mostly re.
For polyatomics usually r0 is obtained from rotational spectra.
For polyatomics usually only a partial description of the geometry can be derived
from the rotational constants, so assumptions are made in determining the geometry,
such as all CH bonds in a methyl group have the same length.
Rules for chemical formula
- Enter a sequence of element symbols followed by numbers
to specify the amounts of desired elements (e.g., C6H6).
- Elements may be in any order.
- If only one of a given atom is desired, you may omit
the number after the element symbol.
- Parentheses may be used to group atoms.
- Multiple specifications for an atom will be added.
This means that CH3(CH2)4CH3 will be
treated the same as C6H14.
- To specify one or more of a given atom, use a
question mark (?) after the element symbol.
- To specify any number (including zero) of given
element, use an asterisk (*) after the element symbol.
- A comma delimited list of several species may be entered.
Species in the CCCBDB
- Mostly atoms with atomic number less than than 18 (Argon). A few have Se or Br.
- Six or fewer heavy atoms and twenty or fewer total atoms.
Versions 8 and higher have a few substituted benzenes with more than six heavy atoms.
Versions 12 and higher have bromine-containing molecules.