Methods for computing nuclear magnetic resonance chemical shielding in large systems. Multiple cluster and charge field approaches
✍ Scribed by Angle C. de Dios; Eric Oldfield
- Publisher
- Elsevier Science
- Year
- 1993
- Tongue
- English
- Weight
- 829 KB
- Volume
- 205
- Category
- Article
- ISSN
- 0009-2614
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✦ Synopsis
Ab initio calculations show that additivity of the intermolecular shielding exists in a model system consisting of fluorobenxene interacting with hydrogen fluoride molecules, C&F-( HF),, where n= l-5. These results indicate that it should be possible to perform chemical sbieldiig calculations on a large system by dividing it into a series of smaller clusters. For M atoms divided into M/N clusters of N atoms, the time savings for large Y is on the order of M3/ I 6N3, a time savings of f* 60 for M= 100, N= 10. We demonstrate the feasibility of using point charges to model long-range electrostatic field effects on shielding by comparing the results of full ab i&o calculations with those obtained by using point charges to represent the HFmolecules in the C&F-( HF), clusters. This comparison shows generally gocd agreement between the two approaches so long as the point charges are r 2.5 A from all the atoms in the molecule to which the nucleus belongs, a situation which should pertain for many macromolecules.
Addition of 1000 point charges to the C6HsF system increased computational time by only 50% and appears to offer promise for investigations of chemical shielding in proteins and nucleic acids, where both short-range (electronic) and longer-range (eleetrostatic field) effects may be important.