Section: 9 | Structure of Free Molecules in the Gas Phase |

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STRUCTURE OF FREE MOLECULES IN THE GAS PHASE

David R. Lide

These tables give information on the geometric structure of selected 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 distances and angles. The tables are restricted to molecules with conventional 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 isotopic 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 regions, 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 molecules, even in their lowest energy state, the definition of interatomic 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 forces between the two atoms. All other measures represent some form of average, generally complex, over the vibrational motions. Because 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 r₀, 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 r₀ 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 (Ref. 1) and in Refs. 4 and 5.

Unless otherwise specified, distances and angles given in this table are r₀ 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, r₀, 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 given here to four or five decimal places, but little chemical significance can be attached to differences beyond the third decimal place.

Table 1 covers molecules that do not contain carbon. Column definitions for Table 1 are as follows.

Column heading	Definition
Mol. form.	Molecular formula of compound; because many of the entries are free radicals or other transient species whose systematic chemical names are unfamiliar, the listing is in order of chemical formula
Name	Chemical name; shown in Online Edition only
Structure	Description of structure; either by words, point group of equilibrium geometry, or structural diagram
Bond/Angle 1	First bond distance or angle described
Value 1	Value of first bond distance (in angströms) or angle (in degrees)
Bond/Angle 2	Second bond distance or angle described
Value 2	Value of second bond distance (in angströms) or angle (in degrees)
Bond/Angle 3	Third bond distance or angle described
Value 3	Value of third bond distance (in angströms) or angle (in degrees)
Method	Method used for structure determination; see abbreviations and discussion above

Table 2 lists carbon-containing molecules. Column definitions for Table 2 are as follows.

Column heading	Definition
Name	Name of molecule; listed alphabetically; name given in boldface above one or more structures
Structure	Description of structure; either by words, point group of equilibrium geometry, or structural diagram
Bond/Angle 1	First bond distance or angle described
Value 1	Value of first bond distance (in angströms) or angle (in degrees)
Bond/Angle 2	Second bond distance or angle described
Value 2	Value of second bond distance (in angströms) or angle (in degrees)
Bond/Angle 3	Third bond distance or angle described
Value 3	Value of third bond distance (in angströms) or angle (in degrees)
Method	Method used for structure determination; see abbreviations and discussion above

The contributions of Kozo Kuchitsu in preparing an earlier version of this table and in giving advice on the new version are gratefully acknowledged.

References

Landolt-Börnstein Numerical Data and Functional Relationships in Science and Technology, Springer-Verlag, Berlin. The following volumes 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.
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. [https://doi.org/10.1063/1.555605]
Huber, K. P., and Herzberg, G., Molecular Spectra and Molecular Structure IV. Constants of Diatomic Molecules, Van Nostrand Reinhold, London, 1979. [https://doi.org/10.1007/978-1-4757-0961-2_2]
Hargittai, M., “Molecular Structure of Metal Halides,” Chem. Rev. 100, 2233-2301, 2000. [https://doi.org/10.1021/cr970115u]
Harmony, M. D., and Berry, R. J., Struct. Chem. 1, 49, 1989. [https://doi.org/10.1007/BF00675784]

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TABLE 1. Structures of Non-Carbon Molecules: Bond Distances in Å and Angles in Degrees

Formula	Name	Structure	Bond/Angle 1	Value 1	Bond/Angle 2	Value 2	Bond/Angle 3	Value 3	Method
Continued on next page...
AgBr	Silver(I) bromide		Ag—Br (r_e)	2.3931					MW
AgCl	Silver(I) chloride		Ag—Cl (r_e)	2.2808					MW
AgF	Silver(I) fluoride		Ag—F (r_e)	1.9832					MW
AgH	Silver(I) hydride		Ag—H (r_e)	1.617					UV
AgI	Silver(I) iodide		Ag—I (r_e)	2.5446					MW
AgLi	Silver lithium		Ag—Li	2.41					UV
AgO	Silver(II) oxide		Ag—O (r_e)	2.0030					UV
AgOH	Silver(I) hydroxide	bent	Ag—O	2.016	O—H	0.952	∠HOAg	108.3^a	MW
AlBr	Aluminum monobromide		Al—Br (r_e)	2.295					UV
AlBr₃	Aluminum bromide	D_3h	Al—Br	2.221					ED
AlCa	Aluminum calcium		Al—Ca	3.148					UV
AlCl	Aluminum monochloride		Al—Cl (r_e)	2.1301					MW
AlCl₃	Aluminum chloride	D_3h	Al—Cl	2.063					ED
AlCo	Aluminum cobalt		Al—Co	2.283					UV
AlCu	Aluminum copper		Al—Cu	2.339					UV
AlF	Aluminum monofluoride		Al—F (r_e)	1.6544					MW
AlF₃	Aluminum fluoride	D_3h	Al—F	1.633					ED
AlH	Aluminum monohydride		Al—H (r_e)	1.6482					UV
AlI	Aluminum monoiodide		Al—I (r_e)	2.5371					MW

^aAssumed value.

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