W. J. Balfour University of Victoria Victoria. B.C. V8W 2 ~ 2 Canada
II
Loosely-Bound Diatomic Molecules
I t is usual for most freshman courses in chemistry to introduce the concept of covalent honding through a discussion of diatomic molecules. The common approach presents the simple molecular orbital treatment of the homonuclear diatomic molecules of hvdroeen. helium and the elements of the first row of the periodic tablewhere the relative hond orders of the molecules can be ~ r e d i c t e dfrom the number of electrons and the bonding o i antibonding characteristics of the various molecular orbitals. Such an approach provides a straightforward explanation for the nonexistence of He2 and Be2 as stable hound species. (Many bound excited electronic states of He2 are known). It is natural to assume that a similar situation holds true for the other rare gases and the other alkaline earth elements. Indeed a statement to the effect that the Nez molecule does not exist appears in most introductory chemistry textbooks. The purpose of the present communication is to draw attention to the fact that, over the past decade, careful spectroscopic studies have established the existence of hound rare gas (e.g. Nez, Ar2) and alkaline earth (e.g. Mg2, Cad diatomic molecules. In agreement with the predictions of simple molecular orbital theory the ground electronic states of the molecules are verv weaklv hound relative to an average - twoelectron covalent hond. However, in other respects these molecules are well behaved and have been characterized in terms of equilibrium bond length, vibrational frequency, and dissociation energy. A summary of their molecular parameters is given in the table. The spectroscopic data contain valuable information on the strength and variation with distance of interatomic (van der Waals) attractive forces. Rare Gas Diatomics The spectra of the diatomic species are observed in absorption with the rare gas cooled to liquid nitrogen temperature. The molecular features occur close to, and to longer wavelengths of, the respective atomic resonance lines which lie in the vacuum ultraviolet region of the spectrum. Therelationshi~between the atomic and molecular transitions mav Iw s w n irom Figure 1. The simple mulertllnr orhitnl dtwriptic~nso i the yruund and lira[ (4wtronic.ally excited states are ground state: . . . . (a.,)2(a.,)4((a,p*)4(cnp')2 excited state: . . . . (an.)2(s,D)4(an~)4(anp*)'(rn+~,a)' where antibonding character is denoted by an asterisk. The
Spectroscopic Data on the Ground Electronic States of some Loosely-Bonded Homonuclear Dlatomlc Molecules. Molecule Nez Are Kr? Xe.
Can Mg2
270
AG(%) (cm-')
r&m)
0.31 0.376
D,(cm-')
13.7 25.7 21.7 19.9
[0.4018
[0.44]= 0.369 0.428
47.9 62.8
300
Reference
174
(1)
84.8
(2) (3) (4) (5) (6)
126.8
185.2 404.1 loso I 150
340
380
WAVELENOTH (NM) F i i 2. Low dlylasion abwvptim s m ofMg uapa. With inoearirg hmaa, temperature, indicetedat theright, the pressure of Mg increaser and gives me rapid increase in me Mg2bands in the 320-370 nm region.
much deeper potential energy well of the excited state is a reflection of the fact that.. w.o n electronic excitation. one electron is promoted from an antibonding to a bonding orbital. The different bond strengths give the two states quite different equilibrium internuclear distances so that the electronic transition in the molecule is accompanied by considerable vibrational band structure. An analysis of the vibrational and rotational spacinas yields data from which the shapes of the respective poten6alenergy curves can be derived. Alkaline Earth Diatomics A situation analogous to that of the rare gases occurs in the vapors of the alkaline earth metals. The electron configurations are ground state: . . . . (a,,.)2(o,.*)2 excited state:. . . . (r~,,)~(a.,*)~(r~,,)'
The suectra are found in the near ultraviolet and visible regions. The example of magnesium is shown in Figure 2. As the temDerature increases and the metal vaDor Dressure increases,-the atomic resonance line broadens. he diatomic spectrum is observed on the long-wavelength wing of the atomic line. Similar observations have been made for calcium. Gas phase data on Bez, Srz, and Baz have not been reported although the spectra of these species are known from matrix absorption studies (7,8). INTERNUCLEAR DISTANCE
Figure 1. A schematic representation of the potential energy curves for the ground and first excited elechonic states of homnucleardiatomicvan &r Waais'
molecules. 452 1 Journal of Chemical Education
Bonding The common feature of the molecules discussed ahove is an equal number of bonding and antibonding electrons. The
molecules possess no formal chemical bond; rather the neutral atoms are held together by weak intermolecular interactions between the atoms. The depth of the potential energy well of the ground electronic state is a measure of the strength of the vanher Waals' attractive force between the atoms. Traditionally, experimental estimates of van der Waals' forces have been obtained from hulk properties of gases or from scattering data. The spectroscopic determination of the shapes of the potential energy curves for van der Waals' molecules has provided accurate data for testing theoretical calculations on these long-range forces. has concentrated On homonuclear 'lthough th:ls diatomic molecules, it should be noted that such molecules occupy only a small corner of the expanding field of van der Waals' molecules. In addition to atom-atom complexes one finds atom-molecule complexes (e.g. Ar-FC1, (9). He-s-
tetrazine (10)) and molecule-molecule complexes (e.g. (COzlz (I1)). The reader is referred for further details to recent reviews on the subject (12-14). ~ (11 (2) (31 (4) (51 (6) (7) (8) I91
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Tanaka,Y. andYoshino, K., J . Chem.Phys., 57,2964(19721. Colhourn. E,A.snd Doup1as.A.E., J. C h r m Phys., 65.1741 (1976). Tanska,Y.,Yoshina. K. and Freeman. D.E.,J Chem. Phys., 59,5160 (19731. Freern8n.D. E., Yahino,K.andTsneka,Y.,J . C h e m Phys., 6I.4880(1974). Ballour, W. J.andDoup1ar.A.R.,Cnn J.Phys.. 48,901 (1970). Ballour, W . J.snd Whit1oek.R. F., Can. J . Phys., 53.473 11975). Brum, J. M., Hew& W. D. and Weltner, W., J . Cham. Phys., 62,3122 (1975). Miller, J.C.,Ault,B.S.andAndrrw,L.. J . Chsm. Phy*., 67,2478i19771. Harris. S. J.. Nouiek. S. E., Klomperer. W, and Fdconer, W., J . Chem. P h w . 61.193
(1974). (101 Smal1ey.R. E.. Whartan, L.,Levy,D.H.sndChandler.D.W.. J Chem. Phys.. 68.2467 (1978). (111 Manniek,L..Str~lsnd,J.C.and Welsh,H.L.,con. ~ . ~ h y s49.3056(1971). .. . ,rntemot. A ~ ~ ~~d W i t .11,486(1972). . (121 E W ~ ~ ~ , G .c hEr m (131 E W ~ ~ ~ , G .J Ephya., . . C 54,487 ~ , , (1976). ila) B I ~ @ Y , L B .and ~wing.~. ~ . . ~ nRe". n .~ h y them., s 27.553 (1976).
Volume 56, Number 7, July 1979 1 453
