Infrared and Raman spectra of some acid salts containing crystallographically symmetric hydrogen bonds

Infrared and Raman spectra of some acid saIts containing crystallographically symmetric hydrogen bonds D. HAD& and B. OREL Institute Boris KidriE, University of Ljubljana, 61001, Ljubljana, Yugoslavia and

A. NOVAK Laboratoire de Cbimie Physique du C.N.R.S., 2 rue Hem%Dunant, 94-Thiais, France (ReceirexZ 23 December 1972) Ab&r&--The infrared and Raman spectra of crystalline NaH(CH&OO),, KH(CFsCOO),, KH(CH,COO), and of their denterated analogs have been investigated in the 4000-30 cm-l range. These scid salts contain orystallographicallysymmetric and very short hydrogen bonds. Assignments of bands concerningthe motions of atoms near the hydrogen bond are given. For iustan~e, characteristics values for vwOHO, r,OHO, SOH and yOH in NaH(CH&OO)s are 720, 320, 1540 and 1285 cm-l. There is strong spectral evidence based on selection rules and the v,OHO/ODO ratio suggesting that the hydrogen bond in these acid salts is of the symmetria single minimum type.

A SYM~~~~~ single m~mum hydrogen bond is well established for (FHF)- in KHFa [l], Corresponding OH0 hydrogen bonds may exist in many acid salts of carboxylic acids containing the (COOHOOC)- group. The crystal structures of several acid salts containing very short (2*50_2*4OA) hydrogen bonds have been determined [2] and also the infrared spectra have been investigated [3, 41. In general there are two types of structures termed A and B by SPEAKM_LN [2]. The latter type contains asymmetric hydrogen bonds, i.e. the acid residues are neither symmetry related nor equivalent and the C-O bond lengths are characteristic of ionized and non-ionized carboxyl groups, respectively. In the A type acid salts the center of the hydrogen bond coincides with a crystallographic symmetry element, a center of inversion or a twofold axis, both carboxylic residues thus being equivalent. However, it does not appear possible to ~e~ntiate by means of ~ffraction methods c~stallo~aphically s~rnet~~l hydrogen bonds with a single central minimum of potential energy from those with two narrowly spaced minima and a small barrier in between [5]. Vibrational spectroscopy may give additional information about this problem. In the past we used infrared spectra in predicting A type structures and in difFerentiating these from the B types [4]. We extended this work using also far infrared J. A. IBERS, J. Chem. Phya. 41, 25 (1964). J. C. SPEAKNAN, Chem. Commw7a.32 (1967). D. EL&& and A. NOVAK,NuowoC&n. Sup@. 2,715 (1955). D. IZADZI and A. NOVAK,hfrawd Spectra of, asd E&d~oges Bonding & Some Acid Sat& of Curboq,&c Acids, University of Ljubjlana (1960). [5] W. C. HAMILTON and J. A. IBERS,Hgdrogea 3~~~g ~~So~~8, Benjamin, New York (1968). 1746 [1) [2] [3] [4]

1746

D. HADW,B. ORELand A. NOVAIC

and laser Raman spectroscopy. In this paper, we want to show that the determination of the single minimum type of a orystallographically symmetric hydrogen bond appears possible using mainly the selection rules and isotope effect. From the many examples studied we have chosen three representative ones, sodium hydrogen diacetate, NaH(CH,COO),, potassium hydrogen di-trifluoroacetate, KH(CF,COO),, and potassium hydrogen succinate, KH(CH,COO),. The most important spectral features only will be discussed here while the detailed assignments for each of these compounds will be published elsewhere [6, 71. EXPERIMENTAL The samples of sodium hydrogen diacetate (NaHA),, potassium hydrogen di-trifluoroacetate (KHT,) and potassium hydrogen succinate (KHSu) were prepared following the methods described by SPEAKMAXet al. [S-10]. Their deuterated derivatives, NaDA,, KBT, and KDSu were obtained by repeated exchange with heavy water. The near and mid infrared spectra were recorded over the 4000-200 cm-i range using Perkin-Elmer 225 and 621 spectrophotometers. The far infrared spectra were obtained in the 206-33 om-l region on a Beckman IR 11 spectrophotometer. All the spectra were of solid state as emulsions in Nujol and Fluorolube. The spectra at liquid nitrogen temperature were examined in a conventional low-temperature cell equipped with potassium bromide, cesium iodide and silicon windows for the sample and external windows of aesium iodide, polyethylene and Teflon for the respective regions. The Raman spectra were obtained on a Coderg PHO spectrograph equipped with a double monochromator. The 4880 A line (900 mW) of a CRL Model 52 A+ and the 6328 A line of a Spectr*Physics Model 125 He-Ne laser (70 mW) were used as exciting lines. The samples were examined as crystalline powders sealed in glass tubes. A Coderg Cryocirc Cryostats was used for low-temperature work. RESULTSAND DISCUSSIONS Crystal structures

The crystal structures of NaHA,, KHT, and KHSu are known [8-lo]. Sodium hydrogen diaoetate crystallizes in the cubic system belonging to the space group Ia with 12 NaH(CH,COO), molecules in a Bravais unit cell. The two acetate residues are equivalent being related by a two-fold axis and linked by a hydrogen bond with 0 . . . 0 = 2.444 A [8]. Potassium hydrogen di-trifluoroacetate is monoclinio (12/a) with four molecules in a unit cell. Two equivalent CF,COO groups are joined in a dimer by a hydrogen bond with 0 . . . 0 = 2.435 A lying across the center of inversion [9]. Potassium hydrogen succinate belongs to the same space group (12/a, 2 = 4). There are infInite -CH,COOHOOCCH,- chains and the acid [0] A. Nova, J. Chim. Phya. 69, 1616 (1972). [7] D. HAD& and B. OREL,~ bepublished. [S]J. C. 8PEAKhfAN and H. H. MIUS,J. Chem. Sot. 1164 (1901). [Q]L. GOLI~ and J. C. SPEAKMAN, J. Ohem. Sot. 2630 (1966). [lo]A. M~ADAM, M. CURRIE and J. C. SPEAKBUN, J. Chem. Sot. A 1994 (1971).

Infrared and Raman spectra of some acid salts

1747

residues, related by a twofold axis, are strongly hydrogen bonded (0

...0 = 2-446 8)

PI*

The hydrogen bonds in all these compounds are thus crystallographically symmetric and the proton possibly occupies the sites C,, C, and C, in NaHA,, KHT, and KHSu, respectively. v’dbTUtiO?Ud

U?&dySiS

We shall be dealing here only with a few vibrations of the -COOHOOC- group which are particularly relevant to the problem of the hydrogen bond described approximately as C==O and C-O stretching, OH stretching and bending, and hydrogen bond stretching. The results of the factor group analysis concerning these motions in the investigated crystals are shown in Table 1. These predictions are used only for KHSu crystal containing infinite, hydrogen bonded chains. The site group approximation, however, appears to be sufficient for interpreting the spectra of polycrystalline KHT, and NaHA, containing one proton dimers, at least for the intra-dimer vibrations. Some of the factor group splittings are observed in single crystal spectra with polarixed radiation [ll]. Table 1.

Symmetry

=P$~o), %’

KH(CF,COO),, KH(CH,COO), and HaH(CH&OO)2crysta.la

speaiea and selection rules for

ix.

R

OH

OH

OH

OH0

f f a e

B a f

0 0 11

0 0 11

0 0 1

1 1 00

KH(CH,COO), O,,O ix.

R

OH

OH

OH

OH0

2

f f

e e

0 1

0 1

1 0

B::

a e

f

0 1

0 1

NaH(CH,COO), T&’ i.r.

R

OH

4 3 4

4

c=O

co

1 1 1

1 1 1

c!=o

c-o

1 0

1 1

1 1

01

0 1

1

1

OH

OH

OH0

c=o

G-o

4 E* =il

f f f

8 e B

0 0 2

0 0 2

1 1 1

1 1 1

1 1 3

1 1 3

4 XI

f e

f f

0 2

0 2

1 1

1 1

1 3

1 3

ix.: infrared.R: Remen, f: forbidden,8: active.

The (CF,COOHOOCCF,)- dimer occupying C, site is thus expected to give rise to a Reman (A,) and to an infrared (A,) C=O stretching band and their frequencies should be different. The same is true of the C-O stretching vibrations. The OH stretching and bending modes are only infrared active while the hydrogen bond stretching vibration is expected only in Raman. There are no symmetry restrictions for the activity of the vibrations of the (CH&?OOHOOCCH,)- dimer of the C, site symmetry. [ll]

B. OREL,Thesis,

University

of Ljubljana

(1972).

1748

D.

H&D&,

l3.

OREL and A. NOVAK

The assignment of the bands reported here (Table 2) is based on a detailed analysis of the infrared and Raman spectra of KHSu, KHT,, NaHA,, their OD deuterated derivatives and of some other isotopic species such as NaH(CD,COO)B and NaD(CD,COO), [7]: the results of the infrared dichroism measured in single crystals of KHSu are also used [ 1I]. Table 2. Frequencies and relative intensitiw of some infrared and &man bands due to the symuwtric -COOHOOC- group 60H vaOH0 YC =o Vc-0 yE&H* YOH Compound ix. ix. R ix. ix. R ix. R i.r. R ~(CF~COO)* KD(CF&OO), KH(CH,COO),

800 ~8, b 660 s, b 860 vs, b

1480 m 1630 m

942 m 1363 ms

t

KD(CH,COO), NaR(CH&OO)., NaD(CH,COO),

6lOs,vb 720 vs, b 600 vs, b

1106m 1642 m 1160 m

1016m 1286 8 960 m

1006~ 1289 m 963 mw

130s 130s

320 e 318 s

1790 w 1736vs 1682 B 1640 VB 1638~s 1711 “B 1666 El

1723 m 1726m 1646 ms 1639ms 1664 a 1663 8

1421 m 1437 8 1380 ms

1439 s 1437 B 1407 m

1390ma 1404 s 1400 s

141411~ 1403 8 1402 s

* Center of gravity of the bsnd. j’ Strong ix. bends nets 130 cm-’ &PObelieved to be due to motions of potassium ions.

OH stretching mode. The infrared region above 1800 cm-l where usually the OH stretching vibration appears is void of any bands which could be attributed to this motion (Figs. l-3). A strong and very broad absorption is observed instead with all acid salts between 1300 and 600 cm-l and is assigned to the OH stretc~g mode. The exact frequency of the maximum absorption is difficult to measure because of the breadth, superimposed bands, and trans~ssion areas. In the infrared spectra of NaHA,, KHT, and KHSu cooled to liquid nitrogen temperature there is some narrowing of the YOH absorption and the frequencies of the band centers are estimated near 720, 800 and 850 cm-r respectively (Figs. l-3, Table 2). The analogous YOD bands are near 500, 560 and 610 cm-l, respectively, yielding YOH/ vOD frequency ratios close to 42. The assignment of the infrared absorption near 800 cm-l to the OH is in agreement with neutron inelastic scattering spectra of some acid salts which show a strong shoulder in this region [12]. A study of the infrared dichroism on single crystals of KHSu [ll] supports the assignment of the 850 cm-1 band as an OH stretch. An extinction of this broad absorption is observed when the direction of the incident beam is parallel to the c-axis, i.e. nearly parallel to the 0 . . . H . . . 0 direction [IO], No Raman counterpart is observed in the spectra of either KHT, where POH is forbidden or NaHAc, where it is allowed. In the Raman spectrum of KHSu, however, there is a relatively broad but weak band near 621 cm-1 which disappears on deuteration and a new broad feature near 479 cm-l appears instead. It may be related to the expected OH stretching mode (A,) but its assignment is not certain. Transmission peaks in the OH stretching absorption of acid salts (Figs. l-3) can be explained according to EVANS [ 131 as a resonance type of interaction between broad and narrow vibrational levels. The broad level is in all oases the OH stretching mode while the narrow levels vary from one compound to other. In the case of [12] &I. F. COLLINS, B. C. HAYWOOD snd G. C. STERLING,J. Chma. Whys. 52, 1828 (1976). [13] J. C. EVANS, ~~~~~~~~~. Acta 17, 129 (1961).

Infrared and Raman spectra of some acid salts

1749

t

E

e

Q)

E al

.E t;

z

3000

2000

1600

1200

800

400

200

cm-1

v.

Fig. 1. Raman spectra of: (a) NaH(CH&OO),, (b) NaDC(CH,COO),. Infrared spectra of: (c) NaH(CH&OO),, (d) NaD(CH&OO),. The spectra are of the solid state at liquid nitrogen temperature.

4000

1800

1400

1000 u.

600

200

100~

cm-l

Fig. 2. Raman spectrum of: (a) KH(CF,COO),. Infrared spectra of: (b) KJS(CF,COO),, (c) KD(CFsCOO),. The spectra are of the solid state at liquid nitrogen temperature.

D. HA&I, B.

1750

I 3000

&EL

and A. NOVAE

I 2000

1600

1200 v.

600

400

200

cm-1

Fig. 3. Reman speotra of: (a) KH(CH,COO),, (b) KD(CH,COO),. Infrared spectra of: (o) KH(CH,COO),, (d) KD(CH,COO),. The speotra are of the solid et&e at liquid nitrogen temperature. NELELA,(Fig. 1)

for instance the narrow level is due to the G-C! stretching vibration of the B species (under C, site symmetry). The corresponding Reman band is observed at 912 cm-l and practically coincides with the infrared “negative peak”. This transmission area disappears in the spectrum of NaDA, (the C-C stretch being observed at 895 cm-l) and a new one is observed near 460 cm-l corresponding doubtless to a skeletal bending mode (Fig. 1). Hydrogen bond stretching vibration. The OH0 symmetric stretching (hydrogen bond stretching) vibrations are more dificult to identify than the antisymmetric ones. The most favorable case appears to be the NaHA, crystal (Fig. 1). All the bands above 400 cm-1 could be identified as intramolecular vibrations of the CH,COO group. Methyl torsions only may appear below this limit as far as other intramolecular vibrations are concerned but they were not observed either in Raman or in infrared as shown by a comparison of the spectra of NaH(CH&OO), and NaH(CD,COO), [?I. All the bands observed in the 400 to 33 cm-l region are thus due to intermolecular motions. In the far infrared spectrum the strong doublet near 220 cm-i corresponds to the motion of sodium ions and a weaker band near 130 cm-l probably to a hydrogen bond bending mode. The hydrogen bond stretching band is identified only in the Raman spectrum as a strong and relatively broad band near 291 cm-l thus at the highest intermolecular frequency which is temperature and deuteration sensitive. It shifts to 3 18 cm-l and sharpens up on cooling the crystal to -180°C. The 318 cm-l frequency decreases to 290 cm-l when going

1761

Infrared and Raman spectra of some acid salta

from NaH(CH,COO), to NaH(CD,COO), [7]. This shift is bigger than expected from the mass effect alone and might be explained by coupling between hydrogen bond stretching and a skeletal angle bending (SCCO) mode near 466 cm-l. The assignment of the hydrogen bond stretching frequencies of KHT, and KHSu appears more difficult. This vibration is only Raman active in the former and the highest intermolecular frequency observed at 130 cm-l, may correspond to a hydrogen bond mode. Two hydrogen bond stretching frequencies are expected for the latter, one infrared and one Raman active, but the assignment cannot be made with any certainty because of the numerous bands in the low frequency region. OH bending rrwdw. There are two OH bending modes usually denoted bOH, in plane, and yOH, out-of-plane vibration. The latter is frequently a pure one, i.e. very little mixing with other motions occurs and its frequency is very sensitive towards hydrogen bond strength. For medium strong and strong asymmetric hydrogen bonds, such as found in the crystals of CH,COOH [14], NaHCO, [15] and KHC,O, [16], the yOH frequency increases from 930 to 990 and 1120 cm-l for the 0 . . . 0 distances of 2.61, 2.59 and 2-54 respectively (Table 3). Table 3. Some vOH/vOD and yOH/yOD 0 ...0 distance

isotopic frequency ratios for hydrogen bonds of various strength and type

(A)

YOH (cm-l)

YOH (cm-l)

NaHCO, NaHC,O,.qO KHC,O4 NaH(CH&OO), KH(CF,COO),

2.625 2.696 2.671 2.634 2444 2.436

2875 2400 1800 1500 720 800

923 998 1042 1120 1285

l-31 l-2 1.0 1-o 1.41 1.43

1.36 1.40 l-38 1.39 1.34

KH(CH&!OO),

2.446

850

1350

I.39

1.33

Compound*

WWOOH),

vOH/vOD

yOH/yOD

Ref.

El41

WI E221 WI WI this

PaPer

WI

The OH mode, on the other hand, is usually more or less mixed with one or two vibrations its frequency being less sensitive towards hydrogen bonding. In the acetic acid crystal the strongly coupled BOH and vC-0 vibrations give rise to 1420 and 1300 cm-l frequencies [14]. There is less coupling in NaHCO, [15] and KHC,O, [16] and the predominantly 80H mode occurs at 1410 and 1470 cm-1 respectively. Much the same behaviour is observed for the OH bending modes of the symmetrical hydrogen bonds. The yOH bond is identified as a strong infrared band at 1286 cm-l in the low temperature spectrum of NaH(CH,COO),. It is very temperature sensitive (1266 cm-l at room temperature) and shifts to 960 cm-l on deuteration. Its Raman counterpart is a weak band at 1289 cm-l. The yOH and yOD frequencies of KHSu and KDSu are at 1363 and 1015 cm-l respectively and are observed in absorption and reflection [ll]. The infrared dichoism of a KHSu single [14] M. HAURIE and A. NOVAK, Spectroch+n. Acta 21,1217(1966). [la] A. NOVAK, P. SAUMAQNE andL. D. C. BOK, J. China. Phys. 80,138s [16] J. DE VILLEPIN and A. ~ov~~,&echy Let& 4,1 (1971).

(1963).

1752

D. HADZI, B. OREL and A. NOVAK

crystal shows that the 1313 cm-l band belongs to the A ~ species and thus cannot correspond to a yOH motion. Finally, in the KDT, spectrum the yOD frequency is found at 870 cm-l while its OH analogue is probably hidden in the 1250-1200 cm-r region of the strong C-F stretching bands. This expectation is borne out by the neutron inelastic scattering spectra of KHT, and potassium hydrogen ditrichloroacetate which show the most intense peak around 1200 cm-l [12]. The 60H modes of symmetric hydrogen bonds pose some problems. In the infrared spectrum of NaHA, there are two bands at 1640 and 1542 cm-l which disappear on deuteration and in the spectrum NaDA, there is a new (SOD) band at 1150 cm-l. The 1542 cm-l band yielding an isotopic frequency (60H/60D) ratio of about 1.34 is assigned to the OH fundamental and the 1640 cm-l band must be due to a combination involving vOH or yOH fundamental. The 60H mode is coupled with the C==O stretching vibration since the C=O frequency shifts from 1711 to 1655 cm-l on deuteration (Table 2). In the case of KHT, the OH bending mode appears to be mixed with C=O as well as with C-O stretching motions of the infrared active A, species, since the &=O frequency decreases from 1790 to 1735 cm-i and the YC-0 frequency increases from 1421 to 1437 cm-l when going from KHT, to KDT,. The 60H frequency must be in the 1400 to 1700 cm-l region but it is not easy to identify. In the ATR spectra of KHT, single crystal a band near 1480 cm-l is observed which appears to be a suitable candidate for this mode [ 111. The Raman spectra of KHT, and KDT, are unlike the infrared spectra practically the same. There is no 80H of A, species and thus no coupling can occur with C=O and C-O stretching vibrations of the A, species. The infrared spectrum of KHSu shows two C==O stretching bands at 1682 and 1640 cm-l. The latter is of the A, type and practically does not shift. Only the former shifts to 1636 cm-1 upon deuteration which implies some coupling with OH motion. It belongs thus to the B, species. There is one C==O Raman counterpart at 1645 cm-l which shifts very little on deuteration and is thus assigned to the A, species. The 60H mode itself is identified at 1530 cm-l and its 60D analogue near 1105 cm-l. Spectroscopic

evidence for symmetrical hydrogen bonding

Singh and Wood have calculated the effect of deuterium substitution on the 0 . . . 0 distance in hydrogen bonds. This results in an expansion for both the symmetric and slightly asymmetric double minimum potentials, whereas bonds with single minima contract [17]. It has been shown experimentally that the increase oftheo.. . 0 distance with deuterium replacement is accompanied with an anomalously low YOH/YOD stretching frequency ratio and a high OH/OD out-of-plane bending frequency ratio as, for instance, in acid oxalates [16] and oxalic acid dihydrate. These ratios may vary from the “normal value” of 1.35 down to unity or less for YOH/YOD and from 1.35 up to 1.40 for yOH/yOD (Table 3). In the present examples of crystallographically symmetric acid salts the contrary is observed: the YOH/YOD ratio (v,OHO/ODO) is higher than normal (Table 3) and the yOH/yOD ratio is normal or even lower. [17] T. A.

SINGH

and J. L. WOOD, J. Chew. Rays. 50, 3572 (1969).

Infrared and Raman spectra of some acid salts

1753

It should be emphasized that the high rOH/yOD ratio is not foreshadowed by some effects of kinematic coupling since the normal coordinate analysis of a similar hydrogen diacid anion (Cl,COO~H~OOCCl,) and the potential energy distribution show that these modes are rather “pure” [18]. The observed OH0 antisymmetric stretching frequencies are very low and indeed lower than the OH bending frequencies. This corresponds to the very long O-H distance of about 1.22 A. Finally, in the case of NaHA, where the symmetric and antisymmetric OH0 frequencies are rather well defined (320 and 720 cm-l) the estimated respective force constant K1 and K, differ by an order of magnitude which is much the same as in the case of the (FHF)- ion [l]. For detailed definition of the hydrogen bond potential function more vibrational data are needed. Unfortunately, no combination frequencies of the OH0 vibrations are evident from the spectra. Two additional facts supporting the spectroscopic evidence of the single minimum hydrogen bond in the present complexes do, however, exist. First, the 0 . . . 0 distance in KHT, does not expand on deuterium substitution [19]. Second, the deuteron quadrupole coupling constant in KDT, [20] is very low (555 kHz) and corresponds to that in KD-maleate which is believed to contain a truly symmetrical hydrogen bond [21]. Thus, there is strong evidence in favour of a symmetrical, single minimum hydrogen bond in KHT, and hence in the other two acid salts discussed. However, this is not generally true of acid salts exhibiting analogous spectra since in a number of such cases very low r,OHO/ODO ratios were observed [ll, 221. Acknowledgements-We are grateful to Mr. F. ROMAIN,Mr. F. CVEK and Mrs. J. BELLOCfor assistancein experimental work. Note aoXed in proof After this paper had been submitted for publication the paper by P. J. MILLER, R. A. BUTLER, and E. R. LIPPINCO~ [J. Chepn. Phys. 57, 6461 (1972)] appeared. The experimental results in the latter se well as the assignmentsare different from those presented here and will be dealt with in a separate paper.

[18] D. HA&I, M. OBRA~OVIB,B. ORELend T. SOLXUJER, J. Mol. Struct. 14, 439 (1972). [19] A. L. MCDONALD,J. C. SPEAKM~LN end D. HA&, J. C&m. Sot. Perlcin II, 826 (1972). [20] J. STEPIISNIK and D. HA&I, J. Mol. Struct. 18, 307 (1972). [21] T. CHIBA,J. Chem. Phya. 41, 1352 (1964). [22] J. DE VILLEPINand A. NOVAK,Spectrochim. Acta a7A, 1259 (1971).