| Popis: |
Nuclear quadrupole resonance (NQR) allows evaluation of small modifications of electron density on resonant atoms in different molecules. NQR study at 77 K shows that CF 2 BrCF 2 Br does not form any charge-transfer complexes with DMSO, PhCOMe and Me 2 CO: 79 Br NQR frequencies (ν Br ) are 306.40, 306.62, 306.50 and 306.42 MHz respectively. The insertion of an electron- withdrawing CF 3 -group onto alkyl bromide molecules CF 3 (CH 2 ) n Br ( I , n =1,2,3) leads to increasing ν Br values and consequently to decreasing electron density on the bromine. Lengthening of the hydrocarbon chain between the CF 3 and Br in I lowers the ν Br value from 289.10 ( n =1) to 267.77 ( n =2) and 256.36 ( n =3) MHz. Important information may be received from NQR reduced frequencies U p =ν obs /ν at , where ν obs is the observed halogen NQR frequency and ν at is the uncharged halogen NQR frequency in the X 2 molecule. The sign of the value of U p −1 corresponds to the sign of the total charge on the halogen atom in the molecule. The data in the table show that negative charges on Cl and Br remain in primary, secondary and tertiary perfluoroalkyl derivatives. Only iodine in tertiary perfluoroalkyl iodides may be charged positively. U p −1 U p −1 U p −1 CF 3 Cl −0.306 CF 3 Br −0.215 CF 3 I −0.098 (CF 3 ) 2 CFCl −0.249 (CF 3 ) 2 CFI −0.046 (CF 3 ) 3 CCl −0.224 (NO 2 ) 3 CCl −0.217 (NO 2 ) 3 CBr −0.066 (NO 2 ) 3 CI +0.145 Cl 2 0 Br 2 0 I 2 0 FCl +0.288 ICl +0.329 These results bring into question the reported mechanism of ‘halogenophilic’ reactions which consist of nucleophilic attack on positively charged halogen atoms. |
