Determination of Electric Field Gradients by Ab Initio Calculations and NMR Relaxation Experiments

• Main Authors: Huan-Yi Shen, 沈桓儀
• Other Authors: Jacob Chun-Hsiung Kuei
• Format: Others
• Language: zh-TW
• Published: 1999
• Online Access: http://ndltd.ncl.edu.tw/handle/40182956639724583178

碩士 === 國立成功大學 === 化學系 === 87 === The electric field gradients（efg）in the vicinity of fluorine or chlorine atoms in fluoromethanes and chloromethanes have been obtained by spin-restricted Hartree-Fock（RHF）6-311G** calculations using Gaussian 92 package. It is found that both inductive effects and negative hyperconjugation have to be employed to rationalize the trend of efg variations in fluoromethanes. This conclusion indicates that, from the viewpoint of efg, not only does the negative hyperconjugation exist but also supplies reconciliation for the controversy reported in recent three decades. In other words, the stabilization of methane derivatives due to fluorination should not be explained solely by either one of the above-mentioned effects. For the chlorinated analogues, however, analysis the values of efg reveals that negative hyperconjugation is so insignificant as to be negligible. Furthermore, the so-call shielding effect proposed earlier by physicists is confirmed by natural population analysis or atomic charge analysis. In closely related cyano-contained compounds, therefore, one usually observes that variations of nitrogen-14 nuclear quadrupole coupling constants（NQCC）are virtually invariant toward substituents and can be account for by pi-effects including negative hyperconjugation. This conclusion is supported by Fenske-Hall MO calculations reported for metal carbonyls. Hence, the present study reconfirms the validity of the NMR relaxation method, which uses NQCC（17O）for evaluation of the backbonding electron density a more recently, despite of that the Townes-Dailey assumptions has been questioned in our labs. Measurements of values quadrupolar relaxation reates and NQCC for 35Cl, 37Cl, 2H have also been conducted in this research in order to verify the versatility of NMR quadrupolar relaxation equation and the reliability of calculated efg by ab initio method, respectively. Factors that would affect the value of efg（or NQCC）have been extensively examined and discussed in detail.