Chlorine Substituted Acetic Acids and Salts. Effect of Salification on Chlorine-35 NQR* Serge David3, Michel Gourdjib, Lucien Guibéb, and Alain Péneaub a Laboratoire de Chimie Organique Multifonctionnelle, Bätiment 420. b Institut d'Electronique Fondamentale, Bätiment 220, Universite Paris-Sud, 91405 ORSAY Cedex, France. Z. Naturforsch. 51a, 611-619 (1996); received October 10, 1995 The NQR of a quadrupolar probe nucleus is often used to investigate the effect of substituent in molecules. The inductive effect, based on a partial charge migration along the molecular skeleton is the only one present in saturated aliphatics, the conjugative effect appearing in conjugated molecules, especially aromatics. As the stepwise charge migration mechanism, formerly used to explain the inductive effect, is now believed obsolete, we have wanted to reexamined the case of chlorine substituted acetic acids and salts. The data in literature was extended by observing reso- nances and determining NQR frequencies in several acids and salts. The present analysis of the salification of mono-, di- and tri-chloroacetic acids, which is equivalent to a deprotonation or the substitution of the acid hydrogen by a negative unit charge, shows that a model based on the polarization of the chlorine atom(s) by the carboxyle group is consistent with experimental results: the polarization energy appears to be proportional to the NQR frequency shifts; experimental data show a correlation between the NQR frequency shifts accompanying salification and the variations of the intrinsic acidity measured in the gas phase; this, in turn shows that there is a proportionality between the polarization energy and the variations in the acid free enthalpy of dissociation. From the comparison between fluorine, chlorine, bromine and iodine, it also appears that an alternative mechanism, the polarization of the carboxyl group by the halogen, would be important only in the case of the fluoroacetic acid. Key words: 35C1 NQR; Chloroacetic acids; Substituent effect; Inductive effect; Polarization; Intrin- sic acidity. Introduction At the same time Allen also reported that the chlo- rine NQR in sodium mono-chloroacetate at a fre- In molecules, NQR frequencies depend, via the elec- quency, 34.794 MHz, is much lower than in the acid, tric field gradient tensor, on the electric charge distri- 36.280 MHz. The decrease in frequency observed bution around the resonant nuclei, and they have when passing from the acid to the salt is explained by extensively been used for analyzing bonding and sub- an increase of the ionic character of the chlorine atom, stituent effects. produced by a stepwise migration of the negative elec- Among the first compounds studied in the early tric charge on the carboxyle group. days of NQR spectroscopy are the chloroacetic acids The stepwise charge migration model for the induc- studied by Harry C. Allen in 1952 [1], The chlorine tive effect is now becoming obsolete since theoretical NQR frequencies in the mono-, di- and tri-chloro- calculations, as well as experimental facts, do not sup- acetic acids increase with the number of chlorine port this model [2]. In particular, 13C NMR shows an atoms in the acid. This increase in frequency is ex- electron depletion on the intermediate carbon atom, plained as resulting from the electron attraction of the contrary to the increase expected from the model of a chlorine atom: the substitution of another chlorine charge migration decreasing with increasing distance atom on the C2 carbon atom in the molecule reduces from the inductive charge. the ionic character of the chlorine atoms(s) already Before reexamining the situation, we first tried to present and increases the NQR frequency. get new experimental results in order to have a wider and more reliable set of data as a basis for the discus- sion. We then tried to develop and propose a new * Presented at the XHIth International Symposium on Nu- clear Quadrupole Interactions, Providence, Rhode Island, description, based on the polarization of the chlorine USA. July 23-28, 1995. atom by the carboxyl group, to interpret the down- Reprint requests to L. Guibe. ward chlorine NQR frequency shift observed upon 0932-0784 / 96 / 0500-0611 $ 06.00 © - Verlag der Zeitschrift für Naturforschung, D-72072 Tübingen 35 612 S. David et al. • Effect of Salification on C1 NQR deprotonation when passing from the acid to a salt; Table 1. Average 35C1 NQR frequencies in chlorinated car- more precisely, the salification of an acid can be con- boxylic acids and their salts, at 77 K, as found in literature. sidered as a deprotonation process: Compound vav/MHz References + RCOOH RCOO " + H , Acids monochloroacetic acid 36.280 [5] and, in turn, the deprotonation can be considered as dichloroacetic acid 38.393 [5] the substitution of an elementary negative charge for trichloroacetic acid 40.124 [5] 3-chloropropionic acid 33.953 [6] the hydrogen atom in the acid group. Monochloroacetates Na + 34.794 [5] K + 34.11 [7] Rb + 33.82 [8] Chlorine-35 NQR Dichloroacetates K + 36.525 [7] Let us just remember that the NQR frequency for a Rb + 36.40 [7] I = 3/2 spin is given by the relation (115), Chapt. 3 in [3] Trichloroacetates Li + 39.65 [8] 2\ 1/2 + 1 e'Qq( » Li , H20 39.006 [V] + q = (1) Na 38.89 [8] 2 h V T ++ Mg , 6H20 39.17 [8] Ca+ +, 4 H-,0 39.09 [8] where vQ is the NQR frequency, eQ the chlorine + Cs , xH26 38.11 [9] quadrupole moment, eq the main component of the electric field gradient tensor (efg), q the efg asymmetry parameter and h the Planck constant. In each of the series appearing in Table 1, there is As the asymmetry parameter, q, cannot be easily only a small number of compounds; for example, con- obtained from experiment and its value, when it has sidering sodium salts, the mono-chloroacetate is been measured, appears to be small in the kind of shown but not the di- and tri-chloroacetates are compounds studied, we will make the approximation shown the corresponding anhydrous salts are missing. 1 e2Qq This led us to search for more resonances and the (2) results obtained are presented in Table 2. Unfortunately, some difficulties were encountered. In the frame of the Townes and Dailey model [4] the We could not obtain any complete series: Many value of the electric field gradient, eqmo, , is related to compounds are not commerically available and have the atomic electric field gradient, eqM , and the popu- to be synthetized but some of them are difficult to lations of the different atomic orbitals of the bonding synthetize because they hydrate or are difficult to shell on the chlorine atom as well as the s-p hybridiza- extract from the solvent used for the preparation. tion of the orbitals in the bond direction. Here also, we Several searches were unsuccessful; in particular, no will use an approximation neglecting the hybridiza- chlorine NQR signal was found in potassium tion and the elctric field gradient thus reduces to trichloroacetate, prepared in completely anhydrous conditions, starting from potassium tertbutylate in (3) anhydrous benzene. Several samples give quite a where a is the population of the pz orbital along the broad spectrum limiting the meaning of the frequen- bond direction. The populations of the px and py, cies measured. The NQR spectrum in a given com- orbitals are taken to be 2. pound is generally composed of several lines corre- As a result of relations, (2) and (3), the variations of sponding to chemically and/or crystallographically Av, Aqmol /qal , and Aa, are proportional. nonequivalent sites of the resonant nucleus. In most of the compounds considered in this study, the chlorine NQR spectra contain several resonance lines; in Experimental Results and General Comments Table 1, which shows results found in literature, only average frequencies are reported when several reso- The experimental data available in the literature nances were observed while, in Table 2, where new when this study was started are shown in Table 1. results found in this study are reported, all frequencies 613 S. David et al. • Effect of Salification on 35C1 NQR Table 2. 35C1 NQR frequencies in chlorinated carboxylic 5-chlorovaleric acid 33.546 MHz, acids and their salts, at 77 K, as obtained in the present work. methyl chloride 34.023 MHz, When the spectrum consist of a single line, its frequency is shown in the average frequency column at right. ethyl chloride 32.704 MHz, Compound v/MHz /MHz propyl chloride 32.968 MHz, butyl chloride 33.255 MHz. Acids 4-chlorobutyric acid 32.857 Besides the couple methyl chloride - chloroacetic 5-chlorovaleric acid 33.546 2-chlorosuccinic acid 36.034 acid, with a frequency difference of 2.25 MHz, another Monochloroacetates significant difference, although a little less than 1 MHz, ++ Ba , H20 33.423 33.196 33.310 0.68 MHz, is seen in the case of 3-chloropropionic ++ Ca , 2H20 34.39 34.77 34.58 acid. The increase in frequency observed between 5- Dichloroacetates chlorovaleric and 4-chlorobutyric acids is quite prob- Ba+ + 35.720 36.995 37.590 37.850 37.039 ably due to the corresponding phenomenon observed K + 36.392 36.632 36.512 for other physical properties of nonbranched chlori- + Na 37.0* nated hydrocarbons. An interesting compound for NMe; 35.8* this analysis would have been 3-chloropivalic acid, Trichloroacetates + + but, unfortunately, no resonance was found in this Ba 38.187 38.541 38.625 38.928 compound. 39.226 39.494 38.833 The question may arise of a possible conforma- NMe, 37.504 37.615 37.733 37.841 37.673 tional effect on the frequency in chloroacetic acid. 3-chloropropionate The most favoured conformation for monochloro- Ba+ + 32.68 acetic acid is that in which the carboxyl group is * Central frequency of a broad spectrum extending over more than one megahertz.