Quantifying Weak Hydrogen Bonding in Uracil and 4-Cyano-4'-ethynylbiphenyl: A Combined Computational and Experimental Investigation of NMR Chemical Shifts in the Solid State
详细信息 查看全文
- 作者：Anne-Christine Uldry ; John M. Griffin ; Jonathan R. Yates ; Marta Pé ; rez-Torralba ; M. Dolores Santa Marí ; a ; Amy L. Webber ; Maximus L. L. Beaumont ; Ago Samoson ; Rosa Marí ; a Claramunt ; C
- 刊名：Journal of the American Chemical Society
- 出版年：2008
- 出版时间：January 23, 2008
- 年：2008
- 卷：130
- 期：3
- 页码：945 - 954
- 全文大小：332K
- 年卷期：v.130,no.3(January 23, 2008)
- ISSN：1520-5126
文摘
Weak hydrogen bonding in uracil and 4-cyano-4'-ethynylbiphenyl, for which single-crystaldiffraction structures reveal close CH···O=C and C
CH···NC distances, is investigated in a study thatcombines the experimental determination of 1H, 13C, and 15N chemical shifts by magic-angle spinning (MAS)solid-state NMR with first-principles calculations using plane-wave basis sets. An optimized synthetic route,including the isolation and characterization of intermediates, to 4-cyano-4'-ethynylbiphenyl at naturalabundance and with 13C13CH and 15NC labeling is described. The difference in chemical shifts calculated,on the one hand, for the full crystal structure and, on the other hand, for an isolated molecule depends onboth intermolecular hydrogen bonding interactions and aromatic ring current effects. In this study, the twoeffects are separated computationally by, first, determining the difference in chemical shift between thatcalculated for a plane (uracil) or an isolated chain (4-cyano-4'-ethynylbiphenyl) and that calculated for anisolated molecule and by, second, calculating intraplane or intrachain nucleus-independent chemical shiftsthat quantify the ring current effects caused by neighboring molecules. For uracil, isolated molecule toplane changes in the 1H chemical shift of 2.0 and 2.2 ppm are determined for the CH protons involved inCH···O weak hydrogen bonding; this compares to changes of 5.1 and 5.4 ppm for the NH protons involvedin conventional NH···O hydrogen bonding. A comparison of CH bond lengths for geometrically relaxeduracil molecules in the crystal structure and for geometrically relaxed isolated molecules reveals differencesof no more than 0.002 Å, which corresponds to changes in the calculated 1H chemical shifts of at most 0.1ppm. For the CCH···NC weak hydrogen bonds in 4-cyano-4'-ethynylbiphenyl, the calculated moleculeto chain changes are of similar magnitude but opposite sign for the donor 13C and acceptor 15N nuclei. Inuracil and 4-cyano-4'-ethynylbiphenyl, the CH hydrogen-bonding donors are sp2 and sp hybridized,respectively; a comparison of the calculated changes in 1H chemical shift with those for the sp3 hybridizedCH donors in maltose (Yates et al. J. Am. Chem. Soc. 2005, 127, 10216) reveals no marked dependenceon hybridization for weak hydrogen-bonding strength.
CH···NC distances, is investigated in a study thatcombines the experimental determination of 1H, 13C, and 15N chemical shifts by magic-angle spinning (MAS)solid-state NMR with first-principles calculations using plane-wave basis sets. An optimized synthetic route,including the isolation and characterization of intermediates, to 4-cyano-4'-ethynylbiphenyl at naturalabundance and with 13C13CH and 15NC labeling is described. The difference in chemical shifts calculated,on the one hand, for the full crystal structure and, on the other hand, for an isolated molecule depends onboth intermolecular hydrogen bonding interactions and aromatic ring current effects. In this study, the twoeffects are separated computationally by, first, determining the difference in chemical shift between thatcalculated for a plane (uracil) or an isolated chain (4-cyano-4'-ethynylbiphenyl) and that calculated for anisolated molecule and by, second, calculating intraplane or intrachain nucleus-independent chemical shiftsthat quantify the ring current effects caused by neighboring molecules. For uracil, isolated molecule toplane changes in the 1H chemical shift of 2.0 and 2.2 ppm are determined for the CH protons involved inCH···O weak hydrogen bonding; this compares to changes of 5.1 and 5.4 ppm for the NH protons involvedin conventional NH···O hydrogen bonding. A comparison of CH bond lengths for geometrically relaxeduracil molecules in the crystal structure and for geometrically relaxed isolated molecules reveals differencesof no more than 0.002 Å, which corresponds to changes in the calculated 1H chemical shifts of at most 0.1ppm. For the CCH···NC weak hydrogen bonds in 4-cyano-4'-ethynylbiphenyl, the calculated moleculeto chain changes are of similar magnitude but opposite sign for the donor 13C and acceptor 15N nuclei. Inuracil and 4-cyano-4'-ethynylbiphenyl, the CH hydrogen-bonding donors are sp2 and sp hybridized,respectively; a comparison of the calculated changes in 1H chemical shift with those for the sp3 hybridizedCH donors in maltose (Yates et al. J. Am. Chem. Soc. 2005, 127, 10216) reveals no marked dependenceon hybridization for weak hydrogen-bonding strength.