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Experimental Insights into the Electronic Nature, Spectral Features, and Role of Entropy in Short CH3•••CH3 Hydrophobic Interactions

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  • Sounak Sarkar
  • Sajesh P. Thomas
  • ,
  • Lokeswara Rao Potnuru, Indian Inst Sci, Indian Institute of Science (IISC) - Bangalore, NMR Res Ctr
  • ,
  • Alison J. Edwards, ANSTO, Australian Nuclear Science & Technology Organisation
  • ,
  • Arnaud Grosjean, Univ Western Australia, University of Western Australia, Sch Mol Sci
  • ,
  • Krishna Venkatachala Ramanathan, Indian Inst Sci, Indian Institute of Science (IISC) - Bangalore, NMR Res Ctr
  • ,
  • T. N. Guru Row, Indian Inst Sci, Council of Scientific & Industrial Research (CSIR) - India, Indian Institute of Science (IISC) - Bangalore, Solid State & Struct Chem Unit

Hydrophobic interactions are often explored in solution-state aggregation of molecules. However, an experimental electron density description about these interactions is still lacking. Here, we report a systematic study on the electronic nature of methyl center dot center dot center dot methyl hydrophobic interactions in a series of multicomponent crystals of biologically active molecules. Charge density models based on high-resolution X-ray diffraction allow the visualization of subtle details of electron density features in the interaction region. Our study classifies these interactions as atypical group center dot center dot center dot group interactions in contrast to sigma-hole interactions, which are stabilized by the minimized electrostatic repulsion and maximized dispersion forces. For the first time, we quantified the solid-state entropic contribution from the torsional mode of the methyl groups in stabilizing these interactions by thermal motion analysis based on neutron diffraction as well as variable-temperature crystallography. The carbon atoms in methyl center dot center dot center dot methyl interactions show a unique upfield chemical shift in the C-13 solid-state NMR signal.

Original languageEnglish
JournalJournal of Physical Chemistry Letters
Pages (from-to)7224-7229
Number of pages6
Publication statusPublished - Nov 2019

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