structure of free moLecuLes in the gas phase
This table gives information on the geometric structure of se-
lected molecules in the gas phase, including the overall geometry,
interatomic distances, and bond angles . The molecules have been
chosen to provide data on a wide variety of chemical bonds and
to illustrate the influence of molecular environment on bond dis-
tances and angles . The table is restricted to molecules with con-
ventional covalent or ionic bonds, but it should be pointed out
that structure data on many loosely bonded complexes of the van
der Waals type have recently become available . The references
below contain data on many molecules that are not included here
and give additional information such as uncertainties and isoto-
pic variations .
The two techniques for gas phase structure determination are
spectroscopy and electron diffraction . The following codes are
used to indicate the method used for each set of data:
ED – Gas phase electron diffraction
MW – Microwave spectroscopy, including both measurements
in bulk gases and molecular beams
IR – Infrared spectroscopy
R – Raman spectroscopy
UV – Electronic spectroscopy in the ultraviolet and visible re-
gions, including fluorescence measurements
ESR – Electron spin resonance .
In some cases data from two sources have been combined to
derive the structure; these are labeled by “ED, MW,” for example .
Because of the internal vibrations that are present in all mol-
ecules, even in their lowest energy state, the definition of inter-
atomic distance is not a simple matter . The ideal measure is the
equilibrium distance in the hypothetical non-vibrating state,
designated by r
e
. This is the value of the separation of the atoms
at the minimum of the potential function that describes the forc-
es between the two atoms . All other measures represent some
form of average, generally complex, over the vibrational mo-
tions . Since the potential function is asymmetric and less steep
at distances beyond the potential minimum, the average distance
is normally greater than r
e
. Distances determined by electron
diffraction (ED) represent an average over all vibrational states
that are populated at the temperature of the measurement; the
most common measure is designated r
g
. Distances determined
by spectroscopy (MW, IR, R, or UV) through measurements on
the ground vibrational state of the molecule, designated by r
0
,
describe some form of average, not easily defined, over the zero-
point vibrations . Another measure that is frequently used in
microwave spectroscopy is the “substitution” distance r
s
, which
is operationally defined through a series of measurements on
different isotopic species . In simple cases, r
s
often lies between
r
0
and r
e
and is therefore a closer approximation to r
e
. Several
other types of averages have been used; good discussions can be
found in Volumes II/25 and II/28 of the Landolt-Börnstein series
(Reference 1) and in References 4 and 5 .
Unless otherwise specified, distances and angles given in this
table are r
0
values if the method is spectroscopic and r
g
values if
the method is electron diffraction . When given, equilibrium and
substitution distances are designated by r
e
and r
s
, respectively .
Many interatomic distances and angles calculated by ab initio
techniques have been reported in the recent literature . However,
it should be emphasized that all data in this table are obtained
from direct experimental measurements . In a few cases, ab initio
calculations of vibration-rotation interaction constants have been
combined with the primary experimental measurements to derive
r
e
values in the table .
The number of significant figures in the values is an indication
of the precision of the measurement; thus a distance quoted to
three decimal places is probably reliable to about 0 .005 Å or better .
However, discrepancies between r
e
, r
0
, and r
g
values for the same
bond are often the order of 0 .01 Å because of vibrational averaging
considerations, so care must be taken in comparing bond distances
in different molecules . Some distances in simple molecules are giv-
en here to four or five decimal places, but little chemical significance
can be attached to differences beyond the third decimal place .
The table is presented in two parts: Part A covers molecules that
do not contain carbon while Part B lists carbon-containing mol-
ecules . Because many of the entries in Part A are free radicals or
other transient species whose systematic chemical names are unfa-
miliar, the listing in Part A is in order of chemical formula . Part B is
ordered by name . In both parts the second column gives informa-
tion on the overall configuration of the molecule, often indicated by
the point group of the equilibrium geometry . Columns 3 through 8
give the values of the bond distances and angles, and the last column
indicates the experimental method . Distances are given in Å units,
where 1 Å = 10
-10
m or 0 .1 nm . Angles are given in degrees .
The efforts of Kozo Kuchitsu in preparing an earlier version of
this table and in giving advice on the new version are gratefully
acknowledged .
references
1 . Landolt-Börnstein Numerical Data and Functional Relationships in
Science and Technology, Springer-Verlag, Berlin . The following vol-
umes are in the series Structure Data of Free Polyatomic Molecules:
II/7, 1976
II/15,1987
II/21, Supplement to II/7 and II/15, 1992
II/23, Supplement to II/7, II/15, and II/21, 1995
II/25A, Inorganic Molecules, 1998
II/25B, Molecules Containing One or Two Carbon Atoms, 1999
II/25C, Molecules Containing Three or Four Carbon Atoms, 2000
II/25D, Molecules Containing Five or More Carbon Atoms, 2003
II/28A, Inorganic Molecules, 2006
II/28B, Molecules Containing One or Two Carbon Atoms, 2006
II/28C, Molecules Containing Three or Four Carbon Atoms, 2007
II/28D, Molecules Containing Five or More Carbon Atoms, 2007 .
2 . Harmony, M . D ., Laurie, V . W ., Kuczkowski, R . L ., Schwendeman,
R . H ., Ramsay, D . A ., Lovas, F . J ., Lafferty, W . J ., and Maki, A . G .,
“Molecular Structure of Gas-Phase Polyatomic Molecules Determined
by Spectroscopic Methods”, J. Phys. Chem. Ref. Data, 8, 619, 1979 .
3 . Huber, K . P ., and Herzberg, G ., Molecular Spectra and Molecular
Structure IV. Constants of Diatomic Molecules, Van Nostrand
Reinhold, London, 1979 .
4 . Hargittai, M ., “Molecular Structure of Metal Halides,” Chem. Rev . 100,
2233-2301, 2000 .
5 . Harmony, M . D ., and Berry, R . J ., Struct. Chem . 1, 49, 1989 .
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