Hydrogen bonding in potassium hydrogen maleate and some simple derivatives studied by inelastic neutron scattering spectroscopy

Chemical Physics 64 (1982) 151-157 North-Holland Publishing Company

HYDROGEN

BONDING

IN POTASSIUM

AND SOME SIMPLE DERIVATIVES SCATTERING

HYDROGEN

STUDIED

MALEATE

BY INELASTIC

NEUTRON

SPECTROSCOPY

John TOMKINSON Neutrotz Divisimr. Rtl:herford a?td Appleto?! Laboratories. CMton, Oxon. OXI 1 OQX. UK and Ian J. BRAID, Joseph Department of Ckmistry,

HOWARD and Thomas C. WADDINGTON

f

Ur~iversityof DurJzanz.SouzJz Road. Durkzm, DHI 3LE, UK

Received 15 June 1981 inelastic neutron scattering spectra (300-2500 cm-‘) of KH(CHCOZ)2, KD(CHCOZ),, KH(CDC02)2 and KH(CHC02 - CCICO2) have been obtained and the vibritions of the hydrogen bond, wirh the exception of vz(OHO), BSsigned. This is the first assignment of these vibrations in a centrosymmerric intramoleculrtrly hydrogen bondsd complex. u&OHO) \vas found to be heavily mixed and to give rise to a strong doublet in the INS spectrn.

1. Introduction Over the past ten years there have been significant advances in both our understanding and the assignment of the spectra of hydrogen bonded compounds [ 11. Despite this there has not been a full assignment of the vibrations of the hydrogen bond in any compound containing a formally symmetric intramolecular hydrogen bond. In particular, recent spectroscopic studies of the maleate ion are conspicuously absent. This probably arises because the intense IR band [2], usually found for antisymmetric stretches ir? the spectra of hydrogen bonded compounds is absent. A possible explanation for this, which involves cancellation of the transition dipole by an induced dipole of equal magnitude in the orbitals of the C=C bond, has been outlined [3]. A normal coordinate analysis has been attempted [4] but has been shown to be inadequate [5,6]. We have recent/y shown that inelastic neutron scattering spectroscopy (INS) can be successfully applied to the identification and assignment of the normal modes of vibration of a hydrogen bond [7]. INS is

particularly sensitive to vibrations which involve proton motion [8] and the advent of the IN1 B spectrometer, located on the hot source at the Institut Laue Langevin, has enabled us to obtain spectra to much higher energy transfers (~2500 cm-t) than was previously routinely possible (=I000 cm-l). We chose to extend our INS studies of hydrogen bonded compounds by obtaining the spectra of some maleates (fig. 1) for three reasons: (i) they are the simplest centrospmmerric, intramolecular strongly hydrogen bonded compounds, (ii) complete assignments of the vibrations of the hydrogen bond have not proved possible using optical spectroscopic techniques and (iii) we wished to extend the INS technique towards

Fk. 1. Schematic diagam As.

0301-0104/82/0000-OOOO/SO2.75

0 1982 North-Holland

of the maIeate ion showin_e the Cz

the study of systemswhich are more complex than those so far reported. To this end we have made use of the unique deuteration properties

of IKS spectroscopy.

The intensity

of an IXS band in our spectra is approx-

planes of symmetry, however it is only slightly nonplanar and we shall adopt the pseudo symmetry of C, for the point group, hence: r,,

= 19A’ + &1”.

inlately given b>- [7{: IA = ~~~&~(u”(~~)~ esp[-2TVH(F,)]Z(v).

where the symbols arc explained most important

factor

of present discussion.

(I)

in the appendix.

in this equation. is the cross section

for the purposes

3.1. Preparation of sanzples

oH. The

values for hydrogen and deuterium are 80 and 2 barns [9], respectively. Thus pure modes involving displacement of deuterons will normally be absent from our spectra which shall be dominated by modes involving the displacements of the remaining hydrogen atoms.

2. hlolecular

3. Experimental

The

symmetry

Potassium hydrogen maleate (KHTM)is known to a strong hydrogen bond [lo] and Pederson and Leby’s two-dimensiona! diffraction pattern showed it to be both centrosymmetric and intramolecular [ll j _ The most recent, detailed diffraction results on KHM [I?]. concern principa!ly rhc symmetry of the (CCO& frame. The crystal space group is Pbcm(Di$, with four molecules to the unit cell. The moIecuIar geometry was reasonably represented by the point group C,,. This was confirmed by MailloIs [5] for KHM in aqueous solution. In the crystal, however [6], the lower symmetry of C, was preferred, with the ion being slightly gauche. The factor group splittings used by MaiIIos to draw his conclusions concerning the geometry of the ion in the crystal are beyond our moderate spectral resolution_ Conse_quently we &II adopt the simpler point group (Czy) for KHM and thus:

contain

r~ib=10_4t+~Z+9g~+4Bt. We are fortunate to have a neutron diffraction study of potassium monochloromaleate [ 131. The work was specifically undertaken to determine the effect of the chlorine atom, which renders the ring asymmetric, on the hydrogen bond. SurprisingIy the hydrogen atom, remains almost centralised aIong the O-O axis.

(Explanations for this involve intermolecular contacts [I41 .) The space group is Pcbn(Dg) with ei&t molecules to the unit cell. The molecule has no axes, nor

KH(CHCO?), was prepared by reacting KHCO, with maleic acid, and KD
153

J. Tomkinson PI at-/INS spectra of hydrogen m&ate and some nmple derivatives

resolution functions which are approximately gaussian. The spectral frequencies as stated are accurate to within zk10 cm-‘. Further details of this technique are given elsewhere [8,9].

4. Results and discussion 4.1. The CH vibrations In the INS spectrum of KDM (fig. 2a) we expect to observe only those normal modes which involve significant motion of the H atoms. The normal modes principally associated with the hydrogen bond will be absent because of the relatively low cross section of deuterons. The full spectrum of KDM should therefore consist of six bands; the in- and out-of-phase components of (a) a(CH) (the in-plane bend),(b) r(CH) (the out-of-plane bend) and (c) v(CH) (the stretch). Of these only the bends occur in our spectral range; both components of 6(CH) have been assigned at =I300 cm-! and those of -&Xl) at =900 cm-’ in KHM [5,6] _ We can therefore immediately assign the KDM bands at 1388 and 1218 cm-l to 6(CH) and those at 871 and 1007 cm-’ to r(CH) (fable 1). These assignments are in good agreeTable 1 Summary

of the INS data ad

assignments

KDhl

(b)

Incickni

2000

1500

1000

500

neutron

aneqykm-:

Fig. 2. inelastic neutron scatteriq ~pectm (77 I;) of (a) KD(CHCO& and (b) KH(CDCO&. o and + denote data collected using the Cu(200) snd Cu(220) monochrom~tor planes. The solid lines are intended to guide the eye.

for the maleates (cm~‘)

KHhID

KHhlCl

330 vst b)

Appros. description

~00

+ 6 (C=COop

a)

species

Mode number for KHhl [5 1

Bl

27

B2 AZ %

22

Bl Al

25 20

Symmetiy

379 m

360 531 597 671

w St vst st

484 w 572 St

653 St 742 vw 847 m

718 st

va,(OHO) + ? rr,s(OHO) + ? -KH) + 6 (C=O)op ? + vas(OH0)

782 m

? + v,,(OHO)

911

1 NH)

605vst 742 vw 871 St 1007 St

879 984 1209

403 m 508 st

976 m 1218 vst

vst

1341 m, sh 1621 vst, br

1218 m 1388 st, br 1588 bw

1322 vw 165 3 vst, br

St

1000 m 1193 vst 1282 vst _

? f y,s(OHO)

165 3 vst, br

6

Y

(OH01

1 WW (OHO)

Bl Al B2

6 15

Al

a) The inclusion of? indicates that the mode is mixed, probably harmonically, with another mode; op: out-of-plane. b)This band was observed us@ the BFD spectrometer on the Pluto reactor at AERE Harwell. The remaining data were obtained using INIB.

J. TomZinson er oi.lIAX spectra of hydrogen maieare and some simple derivatives

152

ment with those of Msillols [5.6] for KHM, except for vz5 (in-phase y(CH)) which he assignT1a_tSIS cm-’ m KDIM. in KHM and which WCassign at ST: cm The assigmnent from the optica work is clearly incorrcct because there IS no INS band at =S IS cm-l in the INS spectra of either Kilhl or KDM. Having assigned v15 in KDM we can see that our data for KHM is in good agreement with rhe assignments of hlaillols (tab!e 1j with the exception of vz5 wf~ich we reassign ik this salt to the IXS band at 879 cm-‘. Ifour assignments of the four bending modes are correct then we may predict the position of y(CH) and S(CH) in KHMCI. In this salt the in-phase and out-of-phase coupling is absent and the split doubtet becomes a singlet. at about thr mean position. Thus in KHRICI we expect y(CH) at (871 + 1007)/2 = 939

cm -’ and&(CH)at(1218+ 1388)/2=1303cm-‘. Two strong bands are observed (fig. 3a) cIose to these positions in the spectrum of KHMCI, at 911 and 1282 and are assigned accordingly. cm -’ rcspecthely, There is. however, another strong INS band that must be explained. This is the band at 605 cm-t, KDM (597 cm-’ KHM). This band was observed in the F&man spectrum of KEM at 597 cm-t, and was assigned as an A, mode [6] _Its inactivity in the IR confirms this as&nment. The mode involves the y(C=O) out-of-plane deformation, and in view of its intensity in the INS spectra it is heavily mixed with the A z component of y(CH). The intensity of this band. compared with the INS band assigned as y(CH) is not surprising. From eq. (i> we can see that whilst the values of WfI(PA) will be similar for the two bands, the Q values are very different. Thus with any reasonable value of W,(V,) the exponential term [eq. (I)] will contribute much more to the intensity of the lower frequency (lower Q) band than to that at higher frequency. The corresponding B, components of y(CH) and -y(C=O) are similarly expected to mix. This mode was assioJled at 370 cm-’ (KHh1) [5] but is placed at -1 . 130 cm m our spectrum. 4.2. OH0 vibmtiorrs

500

1olo 1nacknt

1500

Moo

2500

neutron enagyhl-’

Fig. 3. Inelastic neutron scattering spectra (77 K) of (a) KH:CHCO,)(CClCO,) urd Co)KH(CHC0&. 0 and -+denote data collected using the Cu(200) and Cu(220) nono-

chromator planes The solid lines are intended to guide the eye.

Only those vibrations which Involve displacement of the hydrogen atom bonded to the oxygens will occur in the INS spectrum of KHMD. We wluld therefore predict only three bands, all expected in our spectral range, nameIy the antisymmetric stretch v,,(OHO); the out-of-plane bend, r(OHO); and the in-plane bend, 6(OHO). The acid maleate ion is known to contain a short symmetric hydrogen bond: O-O distance (R(OO)), 2.44 A (KHM) [ 1 l]_ We will use well established correlations between frequency and R(O0) to estimate yiOH0). From Novak’s compilation [ 171 we estimate y(OH0) to occur at 1250 cm-l _The strong band at 1218 cm-’ (KHMD) is therefore immediately assigned to r(OH0). The only other important band to higher wave number is at 1653 cm-’ and must be assigned to 6(OHO). This latter band is very intense, more so than woLLd be expected if it were mixed with other in-plane

vibrations. From these assignments and our INS data we may assign y(OH0) and F(OH0) in KHM and KHMCI as shown in table 1.

J. Tomkinson et al./I_?JSspectra of hydrogetl maieate and some simple derivatives

This leaves us with 2 tot21 of, at least, five unexplained bands in the INS spectrum of KHMD. Overtones and combination bands have only previously been observed as at most weak features in INS spectra. Because of this they have been ignored in our consideration of the principal bands discussed below. There is no single band that can be assigned to v,,(OHO) on intensity grounds alone. The strongest bands, at 508 and 653 cm-‘, must be the principle candidates however. (The corresponding bands occur at 532 and 677 cm-’ in KHM.) It would appear that the most plausible assignments for these bands are to ~~~(0HO)and Y,(OHO) respectively. We note that the observed symmetries of bands al 540 cm-’ (B2) 2nd 675 cm-’ (Al) in the optical spectra of KHM [ 181 are consistent with these assignments. Such an assignment scheme, however, ignores four considerations: (1) the magnitude and constancy of the splitting of the two bands in a range of compounds, (2) the decreasing hydrogen bond strength from KHM to KHMCI, (3) the displacement vectors of the hydrogen in the modes associated with the two bands are approximately parallel (see appendixj and (4) the INS band associated with YJOHO) in a linear centrosymmetric hydrogen bond is expected to be very weak. The separation (&) of the two bands of the doublet is ~145 cm-l. This separation remains constant, within our experimental error, for KHM, KHMD and KHMCl. From optical data 119: it appears that the same may also be true of KHMCl, (the dichloromaleate) in solution. T&s implies that these bands are linked by some underlying effect. Using crystallographic data, to estimate the strain in the hydrogen bonded maleate ion in KHM, KHMCl 2nd N(But)4HMCl,, one finds that the strain order is N(But),IIMC12 > KHM > KHMCl [ 19,201. In the solid state, KHMCI, forms intermolecular hydrogen bonds [ 191. This implies that in KHM the energy of the hydrogen bond compensates for the distortion of the ring while in KHMCl, this is not so. The strength of the hydrogen bond in KHM is therefore expected to be greater than in KI-IMCl. Spectroscopically this can be translated into displacements of r(OHO), vs(OHO) and Y~~(OHO). As a hydrogen bond weakens vs(OH0) 2nd y(OH0) decrease, while v,,(OHO) increases, in fre-

155

quency. In our spectra the doublet has moved from a mean value of 605 cm-l (KHM) to 645 cm-’ (KHMCl). The possibility that the shifts should be considered 2s 532 (KHM) moving to 718 (KHMCI) [bands assigned to v,,(OHO)] and 677 (KHM) to 572 (KHMCI) [bands assigned to Y&OHO)] can be discounted using the other arguments presented in this section. Rather we consider that the two bands of the doublet should be assigned to YJOHO), the average of which moves to higher frequency as the bond weakens. We now consider the implications of the direction of proton dispiacement in v,(OHO) and v,,(OHO). For 2 linear symmetric intermolecular hydrogen bond, Y&OHO) is inactive in INS spectra because both the proton displacement and u_,,~~~ are zero. In an intramolecular hydrogen bonded system, however, v,(OHO) involves motion of the ring. In the case of KHhl v,(OHOj has A1 symmetry and so the proton is constrained to move radially i.e. along the C2 axis. Since we wilI show (in the appendix) that the bands of the doublet invo?tie proton motion in the same direction the assignment of one component to v,(OHO) and the other to v,,(OHO) is not acceptable. Rather both bands must be assigned to the same mode. Consideration of the intensity of the doublet makes an assignment of botiz bands to v,(OHO) inevitable. In v,(OHO) the mean square displacements are dominated by the heavy atoms of the ring consequently the proton displacement is small and any INS band would be of low intensity. We shali now consider the results of the relative intensity calculations. Full details are given in the appendix. The assumptions of the model permit only one band to be assigned to each normal mode. Therefore for the purposes of our calculations we have arbitrarily chosen the lower frequency component (508 cm-‘) of the doublet to represent v~_ The calculated ratio, l.&JT is 1.5/ 1.O while -he measured value is 1 .O/ 1.O. We note, however, that using the measured rots1 intensity of the doublet yields (Ijo + 1c65j)/Iy = 1.6/l .O. which is in good agreement with the calculated value. (GO8 + I,,;) can be regarded 2s a first approximation to the intensity which would have been observed if yBs h2d occurred 2s 2 single band. In view of the assumptions made, too much reliance must not be placed on the exact values predicted by the model calculations. However it can be seen that our assignment of the bands at 508 and 653 cm -I to

156

L Tomkimoll et al./lnS spectra of I~ydrogerl maleare md some simple deriva:ives

vas is in agreement not only \\ith the band intensities but also with their temperature dependence (see appendix).

5. Conchision We have made the first observations, and assignments of the vibrational bands of the intramolecular hydrogen bond in potassium hydrogen maleate. As expected the frequenciss are typical of strongly hydrogen bonded species, but generally at lower values thsn would be expected for intermoIecular hydrogen bonds of simiiar R(O0). v,,(Oh’O) was observed as a “multiplet”. The mixing which results in the multiplet, is probably harmonic in view of the general agreement between the observed intensity of the multiplet and that predicted for a single band. This con:rasts with the results of a similar study of intermolecular hydrogen bonded dicarboxylates [7] in which the observed intensity. Iv,: was one third of that calculated. Spectroscopically the hydrogen bond in KHM is stronger than that in KHMCI. in agreement with previous crystallographic work [ 19,201.

cross section of the hydrogen atom; fiQ is the momentum transfer during the scattering process; Z(P) is the vibrational density of states; (u”(Y,)> is the mean square amplitude of vibration of the nucleus in the normal mode of frequency F& cm-‘; exp[--2iVH(Q] is the &bye-Wailer factor of the hydrogen atom. A detailed discussion of these equations and the significance of-the tensors U and B(iQ is given in earlier papers [7,21] _In the following we consider on& the dispIacement of the hydrogen bonded proton. For the maleates U, at a temperature T, may be expressed as UT = uTr,T f U&T,

(-4.3)

Lvhere Ucr is the sum of the internal mode contributions to the displacements of the proton (in the harmonic approdmation). Likewise C$&- is the sum of the lattice contributions to the displacement of the hydrogen atom. W2 make the approximation that these lattice contributions can be expressed as three orthogonal Debye-like modes,

U&,=iJ-&XT=

a;

0

i0

a;

0

10

0

at I

0 XT,

(A.9

Acknowledgement We would like to thank the SERC for providing neutron beam time and both the SERC and AERE Harwell for the award of a CASE studentship to one of us (I.J.B.). We would also like to thank Professor D. Hadzi for the communication of upnublished optical results on KHM and KDM.

Appendix

5 db,

The intensity (I,) of a band in our INS spectra, due to a normal mode h. of frequency irA is given by [7] :

IA a:HOh!~2(~~))exp[--7_Tt’H(~~)!Z(~,

(A-1)

where

where uH is the incoherent

where U&, hvolves only zero point motions and for example a: is the total zero point contribution of the lattice modes to the displacement of the proton in the direction parallel to the proton displacement in the internal mode “y”. To a good degree of approximation m may be taken as constant over our experimental iJ= temperature range (10 G TG 80 K) [21]. Substituting eqs. (A.3) and (A-4) into (A.2) and differentiating we obtain:

inelastic neutron

scattering

P=TrU& dT

2 UkT

:B(F.J

TrB(FJ

’

From the INS data for KHMD measured at two temperatures (10 and 77 K) we have calculated the values of dbx/dT given in table 2. The close agreement between the db,/dT values for the bands at 508 and 653 cm-’ shows that the internal modes producing these bands involve hydrogen atom displacements which are essentially parallel. The displacement vectors are unlikely to diverge by more than ~15”.

157

J. Tomkinson et al./INS spectrn of izydrogea maleate and some simpIe derivatives Table 2 Summary

of the parameters

used in the c&x&tions

Frequency

Assignment

~as(OH0) + ? ~as(OH0) + ? y(OHO) 6 (OHO)

(cm-‘)

508 653 1218 1653

a) poor the purpose of the calculations

Differences

;(db,/dT

Qi

(‘4-2)

(A-‘)

20.63

0.0330 a)

55.75 78.40

0.0138 0.0102

- db,/dT’) = a,0 - 2;.

Substituting

the relevant

values

(A.6) of db,/dT

from

table

into eq. (A.6) we obtain

r.4

I -”

= 0

I 0

From

o

x 10-5.

(~7,”- 10.65)

0

0

($ - 14.39

our spectrum

of KIiMD,

T=

(A.7)

IO K, We mea-

sure f7/Is to be 1.3/ 1.O. From eqs. (A. I), (A.2) and (A.7) we can obtain an expression for IT/Is which contains ~72as the only additional unknown. Substituting the observed intensity ratio yields: a: = 41.6 A2 K-l. Using this fitted value of n: we can predict I& This we calculate to be 1.5/l .O.

References [l]

R&rive intensity 1x(10 K)ITh(77 K)

db,/dT (AZ I;-’ x

0.86 0.84 0.87 0.92

6.87 6.03 2.61 1.12

used the lower frequency

normal

R. Schuster, G. Zundel and C. Sandorfy eds., Tine hydrogen bond (North-Holland, Amsterdam, 1976). [2] D.Hadzi and S. Bratos, in: The hydrogen bond, eds. R. Schuster, G. Zundel and C. Sandorfy (North-Holland, Amsterdam, 1976) p. 565.

2

___-

~---

(2(i+

we have arbitrarily

db,/dT for different for instance

between

modes may be written:

for RHMD

component

105)

of ~as(OH0).

[3] D. Hadzi and S. Bratos, in: The hydrogen bond, eds. R. Schuster, G. Zundel and C. Smdorfy (North-Holland, Amsterdnm, 1976) p. 604. [4] K. Nakamoto, Y.A. Sarmn, G.T. Behuke, J. Chem. Phys. 42 (1965) 1662. [S] 3. MailloIs, L. Bardet and R. M&gun, 3. Chim. Phys. 66 (1969) 522. [6] I. Maillols, L. Bardet and R. Mar&tan, J. Chim. Phys. 66 (1969) 529. ]7] J. Howard, C.J. Ludman, T.C. Waddington and J. Ton&&son, Chem. Phys. 46 (1980) 361. [8] J. Howard and T.C. Waddington and J Tnmkinson, Chem. Phys. 46 (1980) 361. [9] B.T.M. Willis ed., Chemical applications of thermzl neutron scattering (Osford Univ. Press, London. 1973). [IO] H.M. Cardwell, J.D. Dunitz and LE. Orgel, J. Chem. see. (1953) 3740. [ I1 ] SW. Pederson and HA. Ler>,, J. Chem. Phys. 29 (1958) 948. [ 121 SF. Darow and W. Cochran, _4cta Cryst. 14 (1961) 1250. [ 131 R.D. Ellison and H.A. Levy, Acta Cryst. 19 (1965) 260. [ 141 1. Olovsson, in: Electronic and magnetic distributions in molecules and crysrals, ed. P. Becker (Plenum New York. 1980). [I51 W.G. Bickford, J. Or:. Chem. 22 (1957) 1080. [16] B. Maier ed., Neutron beam facilities at the HFR, Institut Laue Langevin Internal Report (1977). (171 A. Novak, Struct. Bonding 18 (1977) 177. [l&] D. Hadzi, private communication. [19] L. Colic, S. Detoni, D. Hadzi and 8. Orel, Xdv. X101. Relax. Proc. 8 (1976) 56. [20] L. Golic and I. Leban, .4ch Cryst. 836 (1980) 1666. [21] X1.W.Thomas nnd R.E. Ghosh. Mol. Phyr 29 (1955) 1489.