Van der Waals molecule explained

A Van der Waals molecule is a weakly bound complex of atoms or molecules held together by intermolecular attractions such as Van der Waals forces or by hydrogen bonds.^[1] The name originated in the beginning of the 1970s when stable molecular clusters were regularly observed in molecular beam microwave spectroscopy.

Examples

Examples of well-studied vdW molecules are Ar₂, H₂-Ar, H₂O-Ar, benzene-Ar, (H₂O)₂, and (HF)₂. Others include the largest diatomic molecule He₂, and LiHe.^{[2] [3]}

Supersonic beam spectroscopy

In (supersonic) molecular beams temperatures are very low (usually less than 5 K). At these low temperatures Van der Waals (vdW) molecules are stable and can be investigated by microwave, far-infrared spectroscopy and other modes of spectroscopy.^[4] Also in cold equilibrium gases vdW molecules are formed, albeit in small, temperature dependent concentrations. Rotational and vibrational transitions in vdW molecules have been observed in gases, mainly by UV and IR spectroscopy.

Van der Waals molecules are usually very non-rigid and different versions are separated by low energy barriers, so that tunneling splittings, observable in far-infrared spectra, are relatively large.^[5] Thus, in the far-infrared one may observe intermolecular vibrations, rotations, and tunneling motions of Van der Waals molecules. The VRT spectroscopic study of Van der Waals molecules is one of the most direct routes to the understanding of intermolecular forces.^[6]

See also

• Van der Waals radius
• Van der Waals strain
• Van der Waals surface
• –articles about specific chemicals

Further reading

• So far three special issues of Chemical Reviews have been devoted to vdW molecules: I. Vol. 88(6) (1988). II. Vol. 94(7) (1994). III. Vol. 100(11) (2000).
• Early reviews of vdW molecules: G. E. Ewing, Accounts of Chemical Research, Vol. 8, pp. 185-192, (1975): Structure and Properties of Van der Waals molecules. B. L. Blaney and G. E. Ewing, Annual Review of Physical Chemistry, Vol. 27, pp. 553-586 (1976): Van der Waals Molecules.
• About VRT spectroscopy: G. A. Blake, et al., Review Scientific Instruments, Vol. 62, p. 1693, 1701 (1991). H. Linnartz, W.L. Meerts, and M. Havenith, Chemical Physics, Vol. 193, p. 327 (1995).

Notes and References

1. Blaney. B L. Ewing. G E. Van Der Waals Molecules. Annual Review of Physical Chemistry. 27. 1. 1976. 553–584. 0066-426X. 10.1146/annurev.pc.27.100176.003005. 1976ARPC...27..553B.
2. Friedrich. Bretislav. A Fragile Union Between Li and He Atoms. Physics. 8 April 2013. 6. 42. 10.1103/Physics.6.42. 2013PhyOJ...6...42F. free. 11858/00-001M-0000-000E-F3C4-C. free.
3. Book: Joshua Jortner. Advances in Chemical Physics, Photoselective Chemistry. https://books.google.com/books?id=5qYr6E7Y4P8C&pg=PA323. 8 September 2009. John Wiley & Sons. 978-0-470-14313-1. 323–. Van der Waals Molecules (Donald Levy).
4. Smalley. Richard E.. Wharton. Lennard. Levy. Donald H.. Molecular optical spectroscopy with supersonic beams and jets. Accounts of Chemical Research. 10. 4. 1977. 139–145. 0001-4842. 10.1021/ar50112a006. 1977mosw.book.....S. https://web.archive.org/web/20170923193016/http://www.dtic.mil/get-tr-doc/pdf?AD=ADA045167. dead. September 23, 2017.
5. Hutson. J M. Intermolecular Forces from the Spectroscopy of Van Der Waals Molecules. Annual Review of Physical Chemistry. 41. 1. 1990. 123–154. 0066-426X. 10.1146/annurev.pc.41.100190.001011. 1990ARPC...41..123H.
6. Miller. R. E.. Infrared laser photodissociation and spectroscopy of van der Waals molecules. The Journal of Physical Chemistry. 90. 15. 1986. 3301–3313. 0022-3654. 10.1021/j100406a003.