Summary
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
editExamples 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]
A notable example is the He-HCN complex, studied for its large amplitude motions and the applicability of the adiabatic approximation in separating its angular and radial motions. Research has shown that even in such 'floppy' systems, the adiabatic approximation can be effectively utilized to simplify quantum mechanical analyses.
Supersonic beam spectroscopy
editIn (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]
In study of helium-containing van der Waals complexes, the adiabatic or Born-Oppenheimer approximation has been adapted to separate angular and radial motions. Despite the challenges posed by the weak interactions leading to large amplitude motions, research demonstrates that this approximation can still be valid, offering a quicker computational method for Diffusion Monte Carlo studies of molecular rotation within ultra-cold helium droplets. The non-rigid nature of these complexes, especially those with helium, complicates traditional quantum mechanical approaches. However, recent studies have validated the use of the adiabatic approximation for separating different types of molecular motion, even in these 'floppy' systems.
See also
edit- Van der Waals radius
- Van der Waals strain
- Van der Waals surface
- Category:Van der Waals molecules–articles about specific chemicals
References
edit- ^ Blaney, B L; Ewing, G E (1976). "Van Der Waals Molecules". Annual Review of Physical Chemistry. 27 (1): 553–584. Bibcode:1976ARPC...27..553B. doi:10.1146/annurev.pc.27.100176.003005. ISSN 0066-426X.
- ^ Friedrich, Bretislav (8 April 2013). "A Fragile Union Between Li and He Atoms". Physics. 6: 42. Bibcode:2013PhyOJ...6...42F. doi:10.1103/Physics.6.42. hdl:11858/00-001M-0000-000E-F3C4-C.
- ^ Joshua Jortner (8 September 2009). "Van der Waals Molecules (Donald Levy)". Advances in Chemical Physics, Photoselective Chemistry. John Wiley & Sons. pp. 323–. ISBN 978-0-470-14313-1.
- ^ Smalley, Richard E.; Wharton, Lennard; Levy, Donald H. (1977). "Molecular optical spectroscopy with supersonic beams and jets". Accounts of Chemical Research. 10 (4): 139–145. Bibcode:1977mosw.book.....S. doi:10.1021/ar50112a006. ISSN 0001-4842. Archived from the original on September 23, 2017.
- ^ Hutson, J M (1990). "Intermolecular Forces from the Spectroscopy of Van Der Waals Molecules". Annual Review of Physical Chemistry. 41 (1): 123–154. Bibcode:1990ARPC...41..123H. doi:10.1146/annurev.pc.41.100190.001011. ISSN 0066-426X.
- ^ Miller, R. E. (1986). "Infrared laser photodissociation and spectroscopy of van der Waals molecules". The Journal of Physical Chemistry. 90 (15): 3301–3313. doi:10.1021/j100406a003. ISSN 0022-3654.
Further reading
edit- 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).