Posts Tagged ‘Ohio’

Ionizing ultra-strong acids with water molecules.

Sunday, March 15th, 2015
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My previous posts have covered the ionization by a small number of discrete water molecules of the series of halogen acids, ranging from HI (the strongest, pKa -10) via HF (weaker, pKa 3.1) to the pseudo-halogen HCN (the weakest, pKa 9.2). Here I try out some even stronger acids to see what the least number of water molecule needed to ionize these might be.

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Why is the carbonyl IR stretch in an ester higher than in a ketone?

Thursday, February 28th, 2013
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Infra-red spectroscopy of molecules was introduced 110 years ago by Coblentz[1] as the first functional group spectroscopic method (” The structure of the compound has a great influence on the absorption spectra. In many cases it seems as though certain bonds are due to certain groups.“). It hangs on in laboratories to this day as a rapid and occasionally valuable diagnostic tool, taking just minutes to measure. Its modern utility rests on detecting common functional groups, mostly based around identifying the nature of double or triple bonds, and to a lesser extent in differentiating between different kinds of C-H stretches[2] (and of course OH and NH). One common use is to identify the environment of carbonyl groups, C=O. These tend to come in the form of aldehydes and ketones, esters, amides, acyl halides, anhydrides and carbonyls which are part of small rings. The analysis is performed by assigning the value of the C=O stretching wavenumber to a particular range characteristic of each type of compound. Thus ketones are said to inhabit the range of ~1715-1740 cm-1 and simple esters come at ~1740-1760 cm-1, some 20-30 cm-1 higher. Here I try to analyse how this difference arises.

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References

  1. W.W. Coblentz, "Infra-red Absorption Spectra: I. Gases", Physical Review (Series I), vol. 20, pp. 273-291, 1905. http://dx.doi.org/10.1103/PhysRevSeriesI.20.273
  2. J.L. Arbour, H.S. Rzepa, J. Contreras-García, L.A. Adrio, E.M. Barreiro, and K.K.M. Hii, "Silver-Catalysed Enantioselective Addition of OH and NH Bonds to Allenes: A New Model for Stereoselectivity Based on Noncovalent Interactions", Chemistry - A European Journal, vol. 18, pp. 11317-11324, 2012. http://dx.doi.org/10.1002/chem.201200547

Computers 1967-2011: a personal perspective. Part 3. 1990-1994.

Tuesday, July 12th, 2011
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In 1986 or so, molecular modelling came of age. Richard Counts, who ran an organisation called QCPE (here I had already submitted several of the program codes I had worked on) had a few years before contacted me to ask for my help with his Roadshow. He had started these in the USA as a means of promoting QCPE, which was the then main repository of chemistry codes, and as a means of showing people how to use the codes. My task was to organise a speakers list, the venue being in Oxford in a delightful house owned by the university computing services. Access to VAX computers was provided, via VT100 terminals. Amazingly, these terminals could do very primitive molecular graphics (using delightfully named escape codes, which I learnt to manipulate).

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