THE INFRARED ABSORPTION OF SOME CRYSTALLINE INORGANIC FORMATES K. B. HARVEY, B. A. MORROW,^ AND H. F. SHURVELL Department of Chemistry, University of British Columbia, Vancouver, British Colunzbia Received January 2, 1963 ABSTRACT The infrared absorption of some polycrystalline alkali metal and alkaline earth formates has been measured in the region of 500 cm-i to 3000 cm-'. The observed crystal splitting of the formate fundamentals is discussed in terms of a vibrational analysis under the factor group. INTRODUCTION Apart from some Raman studies of aqueous solutions (1, 2, 3) little has been reported concerning the vibrational spectrum of the formate ion. We have therefore undertaken the investigation of the infrared absorption spectra of a number of crystalline inorganic formates. The present report concerns a preliminary survey of the infrared absorption of polycrystalline samples. EXPERIMEKTAL Those formates not directly obtainable were prepared from formic acid and the corresponding carbonate. All materials used were reagent grade and the salts were further purified by recrystallization from aqueous solution. The lithium and potassium salts are quite hygroscopic and a certain amount of care was necessary in their manipulation. The spectra were obtained on a Perkin-Elmer, Model 421, double-beam grating spectrometer. All of the samples were polycrystalline and were suspended, in the usual manner, in KBr pellets. Under the scanning conditions employed the accuracy would be of the order of &2 cm-i and the resolution 2 cm-' or better. RESULTS AND DISCUSSION Assignments - The frequencies of the observed absorption peaks are summarized completely in Tables I and I1 and representative spectra are shown in Figs. 1 and 2. The vibrational TABLE I Infrared absorption of alkali metal formates (frequencies in cm-') Na(HC0a) Na(DC02) Li(HC02) Na(HC02) K(HC02) Na(DC02) R., Hz0 (Fonteyne) R., H20 (Fonteyne) 'Present address: Department of Organic Chemistry, University of Canadian Journal of Chemistry. Volume 41 (1963) 1181 Cambridge, Cambridge, England.
1182 C.\SADIAS JOURNAL OF CHEMISTRX-. VOL. 41, 19G3 TABLE I1 Infrared absorption of some formates of divalent cations (frequencies in cm-') Ca(HC02)z Sr(HCO2)a Ba(HC02). Ba(HC0s)a Ba(DC0?)2 Pb(HC02)z I.R., crjst. I.R., Hz0 FREQUENCY (cm-11 FIG. 1. Infrared absorption spectra of lithium and sodium formates. analysis of Fonteyne (3), employing a Urey-Bradley potential function, was used as a basis for the assignments. For the most part, the assignments are straightforward but some ambiguities exist, especially in the assignment of the overtones and combinations. Thus, two fundamentals, vl and vg, fall in the region 1340 cm-l to 1400 cm-l. In the spectra of the formates of divalent cations, the assignment of these two fundamentals would appear to be straightforward, with two strong, easily resolved absorption bands
HARVEY ET AL.: INORGANIC FORMATE I.R. ABSORPTION 1183 15 m I I I I I 3 000 2 500 2 000 1500 1000 500 FIG. 2. FREQUENCY (crnl) Infrared absorption spectra _of some alkaline earth formates. being observed in each case. The spectrum of lithium formate also exhibits this feature but those of the sodium and potassium salts differ rather markedly in this region. Presumably this results from the different crystal structure of the formates of potassium and sodium (Table 111). Similar ambiguities exist in the assignment of 2v5. On a frequency basis this would appear to be the only plausible assignment, especially for the 2017 cm-l peak of the Na(DC02) spectrum, but in view of the weak fundamental it is surprising that the overtone is observed. That it is seen in every case must be the result of Fermi resonance with the fundamental vz. Such an explanation also provides a means for rationalizing the variations in the difference between the observed and calculated values for the frequency of 2v5. Fine Structure of the AbsorPtion Bands Since the Cqu symmetry of the free formate ion precludes any degeneracy, the fine structure of the absorption due to the formate, internal vibrations may be ascribed to crystal-field splitting. That such fine structure is to be expected can be seen from the factor group analyses summarized in Tables IV, V, and VI. These tables and the correlation table (Table VII) were prepared by the well-known procedure described by
1184 CANADIAN JOURNAL OF CHEMISTRY. VOL. 41, 1963 TABLE I11 Crystal structures of formates studied System Space group Z Lattice constants Comments Li (HCO*) Orthorhombic Vh16(Pnma) 4 a = 6.49, b = 10.01, or C*.? (Pna2,) c = 4.85 - -" \-., - -. Na(HC02) RlIonoclinic Czh0(C2/c) 4 a = 6.19, b = 6.72, Formate and sodium ions c = 6.49, P = 121'42' on 2-fold axes K(HC02) (Not known, presumable similar to structure of sodium formate) Ca(HC02)2 Orthorhombic Dzh15(Pcab) 8 a = 10.16, b = 13.38, All atoms on general c = 6.27 oositions Sr(HC02)z Orthorhombic Dz4(P212,21) 4 a = 6.87, b = 8.75, ~drtnate ions on two c = 7.27 sets of 4-fold general positions Ba(HC02)2 Orthorhombic D24(P212121) 4 a = 6.81, b = 8.91, Formate ions on two c = 7.67 sets of 4-fold general positions Pb(HC02)2 Orthorhombic Dp4(P212121) 4 a = 6.52, b = 8.75, Formate ion on two c = 7.74 sets of 4-fold general positions TABLE IV Factor group vibrational analysis of crystalline Na(HCO2) C2h E Cz i '~h ni ni (T) n, (T') ni (R') n,' ao 1 1 1 1 6 0 2 1 3 b, 1-1 1-1 9 0 4 2 3 a, 1 I - 1 - I 6 1 1 I 3 b" 1-1 - 1 1 9 2 2 2 3 Bhagavantum (4) and Halford (5). In addition to the character table, giving the symmetry of the vibrations, the tables list the following: ni, the total number of optically active vibrations of the symmetry species i under the factor group analysis; nc (T), the number of purely translational modes of symmetry species i; n, (TI), the number of lattice modes of translational origin of symmetry species i; nr (R'), the number of lattice modes of rotational origin of symmetry species i; n,', the number of internal formate vibrations of symmetry species i. The crystal structures on which these analyses are based have been summarized in Table 111. It should be noted that while the number of molecules (2) in the conventional unit cell of sodium formate is four, the primitive cell will contain only two molecules. In the case of the formates of divalent cations the factor group analysis is almost trivial because, in each case, the atoms all lie on general positions. The unit cell of Ca(HC02)2, for example, contains 16 formate ions composed of two sets of 8 crystallographically equivalent units. A set of eight formate ions, each member of which is executing the same normal vibration, gives rise to eight different combinations with the ions vibrating in phase or 180' out of phase relative to one of their number arbitrarily chosen; each combination corresponds to one of the irreducible representations. The other set of ions will exhibit eight identical combinations but, since the two sets are not related by symmetry, in-phase or out-of-phase combinations of the two sets are of the same symmetry species. Thus a single internal formate vibration has 16 components in the spectrum of the crystal. Only vibrations of three symmetry species are active, however, so that, if completely resolved, the infrared spectrum should reveal each internal vibration to be
HARVEY ET AL.: INORGAN IC FORMATE I.R. ABSORPTION NrnNNNNDIC.1 dddridf,dd
1186 CANADIAN JOURNAL OF CHEMISTRY. VOL. 41. 1963 TABLE VI Factor group vibrational analyses of crystalline Ba(HCO?)z, Sr(HCOz)z, and Pb(HC0a)z Dz E C~(Z) CAY) Cz(x) ni ni (TI ni (T') ni (R') nit TABLE VII Correlation table for Na(HC02) Rlolecular group Site group Factor group split into six components, two each of species bl,, bz,, and b3,. Examination of Table I1 shows that in most cases only a doublet is observed, so that one might tentatively assume that each component of the doublet is composed of three unresolved absorption peaks corresponding to vibrations of symmetry bl,, bz,, and bau. Similar considerations apply in the case of the formates which have the space group D24(P212121). Each formate internal vibration will have eight components in the crystal spectrum-but only six of these are infrared active, two each of species bl, bs, and b3. Again, most of the internal vibrations are split into doublets, so that one is tempted to propose that each component of the doublet is composed of three unresolved absorption peaks corresponding to vibrations of symmetry bl, bz and ba In the case of sodium formate each internal formate vibration should give rise to two components, one of which is infrared active. In addition, because the twofold axes of the ions are coincident with those of the crystal, there is a correspondence between the al modes of the free ion and the a, modes of the crystal, and the bl and bz modes of the free ion and the b, modes of the crystal (Table VII). These conclusions would appear to be in agreement with the observed spectrum. It will be apparent from the above discussion that a study of single crystals of these materials using polarized radiation would be of considerable assistance in deciding the origin of the observed fine structure. Such an investigation of barium formate is nearly completed and will be reported upon shortly. In the case of lithium formate, a single crystal study would likely permit specific characterization of the space group and reveal whether or not the formate ions lie on special positions. In addition, because of the light cation, it is probable that many of the lattice modes will be in regions which are experimentally accessible. ACKNOWLEDGMENTS The authors are indebted to Alr. Rudolf Muelchen for his very capable assistance in the construction and maintenance of the apparatus. We wish also to gratefully acknowledge the financial assistance of the Kational Research Council of Canada.
HARVEY ET AL.: INORGANIC FORMATE I.R. ABSORPTION 1187 On a mesuri. I'absorption infrarouge de quelques formiates inorganiques dans I1i.tat polycristallin. La multipliciti. des bands d'absorption fondamentales de l'ion formiate est discuti. en function de l'analyse vibrationale du groupe facteur. REFERENCES 1. J. LECOMTE. Compt. Rend. 208, 1401 (1939). 2. J. LECOMTE. Cahiers Phys. 17, 1 (1943). 3. R. FONTEYNE. Natuurw. Tijdschr. (Ghent), 25, 173 (1943). 4. S. BHAGAVANTAM and T. VENKATARAYUDU. Theory of groups and its application to physical problems. Andhra University, Waltair. 1951. 5. R. S. HALFORD. J. Chem. Phys. 14,8 (1946).