X-H dipoles

X-H dipoles

Postby jwkeller » Mon May 23, 2016 6:18 pm

All,
"Discovering Chemistry w NBOs" has a detailed discussion of formamide dipoles, including bond, lone pair, and molecular (p. 147 ff). It is pointed out that estimating the molecular dipole using only bond dipoles based on atomic charges or electronegativity differences, as is often done in freshman chemistry texts, is incorrect: at a minimum the lone pair contributions must be included.
But there is another obvious difference between naïve bond dipoles derived from atomic charges, and NBO (or NLMO) bond dipoles: that is the orientation of X-H dipoles. NLMO bond dipoles for BH, CH, NH, and OH bonds point toward H (partial neg to partial positive in the chemist's sense of dipoles)! This in spite of the fact that H is less electronegative than B, C, N, and O, and almost always carries a partial positive charge whether Mulliken or NBO. This is a mystery to me - can someone offer a brief, or not so brief, explanation?
In an effort to understand this issue, I have plotted bond and molecular dipoles for several HX and other molecules (including formamide) at my Web-Based Molecular Modeling website (http://tinyurl.com/zstufpz) under "Dipoles Illustrated". These are JSmol pages and so you need Java-enable browser such as Firefox to see them.
Any other comments could be sent to me at jwkeller / at / alaska.edu.
Thanks,
John Keller
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Re: X-H dipoles

Postby jwkeller » Mon May 23, 2016 6:20 pm

Correction already: the chemist's sense of dipole is partial positive to partial negative.
John K.
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Re: X-H dipoles

Postby ericg » Fri May 27, 2016 8:57 am

John,

A. E. Reed and F. Weinhold, "Some remarks on the C-H bond dipole moment," J. Chem. Phys. 84, 2428 (1986) [http://dx.doi.org/10.1063/1.450359] will likely provide some insight here.

Eric
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