Vibrational spectra of crystalline hypophosphorous acid

Journal of Molecular Structure, 33 (1976) 279-288 @Elsevier Scientific Publishing Company, Amsterdam -

VIBRATIONAL ACID

S. DETONI,

SPECTRA OF CRYSTALLINE

279 Printed in The Netherlands

HYPOPHOSPHOROUS

D. HAD21 and B. OREL

Department of Chemistry, University of Ljubljana and Chemical Institute Lju bljana (Yugoslavia)

Boris KidrG,

(Received 5 November 1975)

ABSTRACT Infrared and Raman spectra of polycrystalline and polarized IR spectra of the ac face of H,PO, and D,PO, are reported and analysed with particular respect to hydrogen bonding. The uOH stretching mode shows a large factor group splitting and a negative mass effect on frequency. The spectra of D,PO, show more bands than those of H,PO, (and more than predicted by the factor group analysis) suggesting a lowering of the symmetry. INTRODUCTION

The structure of hypophosphorous acid (HPA) crystals features chain association of the acid molecules connected by very short hydrogen bonds with symmetric potential wells. Diffraction work [I] has left the problem of the “true” symmetry of these bonds. Previous IR investigations of HPA [ 2 were handicapped by the lack of both crystal structure data and experience with very short hydrogen bonds [3-6]. Hence the discrepancy between the assignment of the OH bands and the shortness of the 0 - - - 0 distance is understandable, but requires a re-investigation. Moreover, we thought that the advantages of polarized spectra in giving much more detailed informatior should also be exploited in this very interesting example of extremely short hydrogen bonding in which ionized entities are not involved. EXPERIMENTAL

Anhydrous HPA was obtained from a commercial product (50 p-c., Merck by evaporating the water under vacuum, m-p. 23.5 “C. Deuterated HPA (=HPA,) was obtained by adding DzO to anhydrous HPA and drying as above. This was repeated until the H-bands were minimal. Crystalline films of HPA with the ac face developed were grown between CsBr plates on a cooled metal plate with temperature gradient. The operation was performed in a dry box and the sandwiches were quickly transferred to the cooling unit (RIIGVLT 2 cell). The polycrystalline samples were mulled in a cold room (-15 QC) using a vibrating mill.

280

The polycrystalline samples for recording the far-IR spectra were obtained by cooling liquid films between polyethylene plates. The IR spectra were recorded on a Perkin-Elmer 180 instrument equipped with AgBr wire grid polarizers. The Raman spectra were recorded on a Varian-Cary 82 instrument using the Ar laser line 4852 A. HPA was contained in capillaries cooled by a boil-off stream of nitrogen. It was possible to record only the rather strongly Raman-active vibrations because of the fluorescence of the samples. The latter appears to be due to the slow decomposition of the anhydrous acid. The orientation of the samples frozen between plates was determined by X-ray precession photographs and checked under the polarizing microscope. The darkening appeared at 22” with respect to the direction of growth of the crystals and this was the direction of the a axis; the c axis is orthogonal to this. However, in HPA, the a axis and the darkening are at 46” with respect to the direction of growth, the c axis being again orthogonal. THE STRUCI’URE

AND THE VIBRATIONAL

BACKGROUND

The symmetry of the HPA is orthorhombic, space group P2,212 or Dz with two molecules in the Bravais cell. The molecules aggregate to zig-zag chains through hydrogen bonds of 0 0 distance 2.44 A which are parallel to the a axis. The projections of the 0 - - - 0 vectors on the ab crystal plane in adjoining chains form angles of 64’. Neutron diffractions show the proton to be centered, at least statistically, between the oxygens. The site group of the protons and the oxygens is Ct, other atoms being in general positions. The ab, bc and ac projections of the structure are shown in Fig. 1. The vibrational analysis under the D1 factor group is given in Table 1. The B-type vibrations are IR-active with B, having the Ilc polarization and B3 the lla polarization. All vibrations are Raman-active. The deformation vibrations of the PH, groups are classified by analogy with methylene groups in hydrocarbon chains. One of the space degrees of freedom of the hydrogen bond stretching oOH0 has the same symmetry (Bl) as the translation of the unit cell (T) and is therefore redundant (marked * in Table 1). l

C

O\

o___ _.

0

1 ___o~p\o____“\/

__H

/

/

l

H

(-J_______H_______O

\

l

\

\.

H

____

I

‘P’

1 a

Fig. 1. Structure of hypophosphorous

acid.- Projection on the 010 face.

TABLE

1

Vibrational

B, BZ B,

crystal (D: )

“i

T’

T

rHPOH

oOH0

R’

TOPOH

THPOH

TPOHO

8 8 10 10

1 1 2 2

1 1 1

0 0 1 1

1 1 0 0

1 1 2 2

0 0 1 1

0 0 1 1

1 1 0 0

D:

A

analysis of H,PO,

vOH

60H

A6

0

0

B,6 B,6 B36

0 0 1

0 0 1

nt

PH

vPH

VP0

6OPO

sPH

rPH

w

1

1

1

1

1

0

0

1

1 1 0

1 1 1

1 1 1

1 1’ 0

1 1 0

0 1 1

0 1 1

1 0 0

yOH

tPH

IR

Ran a

Tz TY TX

a a a

RESULTS

The spectra of polycrystalline HPA and HPA, are shown in Fig. 2, and the polarized spectra in Fig. 3, far-IR spectra in Fig. 4 and Raman spectra in Fig. 5. The band frequencies and assignments are given in Table 2. The differences between the spectra of the polycrystalline and both polarized ones are quite impressive, and are mainly due to the absorptions extending over most of the mid-IR region which are very strong in the Ila polarization and .attributed to the OH stretching vibration. This absorption is interrupted by several “transmission windows”. These are usually flanked by rather sharp maxima which create problems in determining the proper band positions and even in obtaining the true number of bands. Careful checking of both types of IR spectra (non-polarized and polarized) against Raman spectra is

200

LOO

600

600

1000

1200

lLO0

1600

1800

2000

25co

3000

3500

LOO0 cm”

Fig. 2. Infrared spectra of polycrystalline deuterated ticid ( . - - ).

hypophosphorous

acid (--

-)

and the

LOO

600

800

1000

1200

ILOO

1600

1800

2000

2500

3000

3500

COO0 cm“

Fig. 3. Polarized IR spectra of hypophosphorous acid (-

50

100

150

200

250

300

350

) and the deuterated acid ( - - - )_

GO0

Fig. 4. Far-IR spectra of polycrystalline hypophosphorous acid (acid ( - - - ).

LSO

cm”

500

525

) and the deuterated

extremely helpful. It is unfortunate that we did not succeed in obtaining single crystals suitable for Raman polarization measurements.

283 00%

lhO0

1200

1000

800

600

400

200

10030

SHIFT p cti’

Fig. 5. Raman spectra of polycrystalline hypophosphorous acid ( - - - ).

acid (-)

and the deuterated

The PH2 vibrations

The &etchings are represented in the spectrum of the polycrystalline sample by three bands near 2450 cm-’ whereas in the polarized spectra only one band appears in each polarization. This is a good check for the orientation of the samples. The assignment of the deformation bands is based on their polarization properties and deuteration shifts. The case of the B,-type PH2 and PD2 scissoring at 1120 and 814 cm-’ appearing strong in the Ilc spectrum is clear, but that of the twisting vibration at 1092 (HPA) and 730 cm-’ (HPA,) is difficult because of the large deuteration shift (-1.50) and the rather high frequency. However, there are no other bands with proper polarization properties which are also in accord with the Raman spectra. Strong couplings of the PH, deformations with PO, and, perhaps, with OH modes, seem to exist, which may lead to abnormal deuteration ratios. The &-type modes, wagging &nd rocking, have a peculiar appearance in the spectrum Ila. In fact, they are associated with the “transmission windows” and flanking absorption maxima near 910 and 800 cm-‘, respectively. These modes are of the same symmetry type as the broad OH stretching absorption and thus they interact by Fermi resonance with the components of the latter. This phenomenon; known as the Evans effect [,7,8], is quite common in the (ii) type spectra of systems with very strong hydrogen bonds 13-6, 91. The present assignment is also supported by the Raman spectra. Another peculiarity is the doubling of the corresponding PD, bands, of importance in the discussion of hydrogen bonding in *A and HPAa.

805 *790 424 s

1086 m

*910

1115 w $1118 1122 w

2445 s 1400 vs, br

WM

400 m

1018 vs

vs vs, sho vs (cf. rOD) s s

643 vs 620 s 420 vs

1065 1035 1000 *932 918 880

412 vs

980 vs 645 s 622 w

730 m

1092 m

943 m

1800s

II@, 1

814 s

772 s 738s

*932

1772 s 1650 vs, br

II@,)

1120 s

1245 m

2468 s

UC@, 1

Polarized spectra

Frequencies of IR and Raman bands of H,( D,)POf

232 s 218 s 120 s

394 s

1015 s 805 s, br

1015 s

1000 s, br 1130 s (B:)

*928

1098 m

409 279 238 230 112

982 635 620 420

vs s s s s

vs vs s vs, sho

1062 s

1100 s, br 1145 s (B,)

765 s

718 s

805 m

“930

1260 1245 1122 m

1798 s 1782 s 1772 s

spectra

2465 s 2452 s 2442 s

Polycrystailine

90 w

410 s 430 m

90 w

992 vs 632 m

1075 s, br

770m

710 m

815 s

1004 vs 800 m

1075 m, sho

918 m

1100 w, sho

1122 vs

Raman spectra

rock PH

vP0 -c sOD

1

rHPOH

aOD0

TOPOH and 6 OPO

1

_I

1

uOH

wag PH

1

twist PH

sPH

1

1rOH

uOD

uPH

1

Assirrnment

285

The PO2 vibrations

Whereas the B,-type stretching appears as a neat, strong band in the Ilc spectrum, the &-type vibration is probably involved in the Evans effect and has to be located between the minima and maxima of absorption in the Ila spectrum between 1038 and 1098 cm-‘. The &-type stretching is not allowed in the absorption of the ac face, but it has to be sought in the spectrum of the polycrystalline sample in the complex band near 1150 cm-‘. In the Raman spectrum there is one PO* stretching band at 1004 cm-’ whereas another is overlapped by the PH2 deformations near 1100 cm-’ and appears clearly after deuteration as a broad band at 1075 cm-‘. Although all four POZ stretching modes are Raman-active they are unlikely to be clarified without polarization measurements. Deuteration appreciably shifts the PO* stretchings to lower frequencies which indicates their coupling to the PH2 deformations and, possibly, with the OH modes. Moreover, there seem to be more bands present in HPA, in the region of the POZ &etchings than correspond to the number of bands in the spectra of HPA. Thus in the Ilc spectrum there is a peak at 980 cm-’ and the Ila spectrum shows peaks at 1065,1035, 1000, 918 and 880 cm-’ (this peak is likely to harbour two modes if its width is compared to that of the band at 980 cm-‘). Bands appear in the Raman spectrum at 1075 cm-’ and 992 cm-‘, and in the IR spectrum of the polycrystalline sample at 1145,1020,982 and 908 cm-‘. The small frequency differences are not meaningful since the true peaks may be distorted by the Evans effect. This is particularly probable of the band at 918 cm-’ considering its shape. The sharp minimum at 930 cm-’ is probably a transmission window, but of unknown origin. The possibility that the in-plane OD deformation moves into this region has to be considered when counting the POZ stretchings and accounting for their deuteration shifts, but there are still too many bands. The reason is likely to be the same as for the doubling of the PH, deformation bands. With respect to the small number of atoms in the molecular unit of HPA and the chain structure, the OPOH and HPOH torsions (7) include relative motions of two neighbor molecules of the same chain and may be treated as external translations (T’) and rotations (R ‘). The frequencies of the torsions will be rather high because light atoms are involved. The bands at 424 cm-’ (Ila) and 400 cm-’ (11~)are in the region attributed to the PO scissoring (2) and the second band is attributed mainly to this mode. The deuteration shift from 400 to 412 cm-’ is attributed to the activation of the oOD0 mode in HPA,. The band at 279 cm-’ which only appears in the spectrum of the deuterated compound is assigned to this mode. The small deuteration shift of the other band (11~)shows the participation of hydrogens in this mode. Symmetry classifications for the lower frequency bands cannot be given without the polarized spectra. They are certainly due to lattice vibrations of the TOPOH and r HPOH character since they do not exist in the spectrum of the liquid film.

286

The OH0

vibrations

In view of the short 0 is expected

l

l

l

0 distance the out-of-plane deformation 7OI-I

in the region 1200-1300

cm-’

[lo].

In fact there is aB,-type

band at 1240 cm-’ which disappears with deuteration. The shoulder at 940 cm-’ in the Ilc spectrum of HPA, might represent the yOD mode. The bond projection on the ac face for the in-plane deformation 60H is only -0.9 a and therefore the absorption is expected to be very weak. Since the region where it would be expected (1400-1600 cm-‘) is covered by the broad OH stretching absorption it is understandable that we do not find the 6 OH band. The possibility that the 6 OD band appears in the region of the POZ stretchings has been mentioned, but no definite assignment can be made. The OH stretching which appears in the liquid film in the shape of three main peaks between 1600 and 2700 cm-’ changes on solidification to the extremely broad absorption which begins well above 2000 cm-’ and extends into the far-IR region. More impressive than the spectrum of the polycrystalline sample is the one Ila, whereas the spectrum Ilc is completely devoid of the broad absorption. Since the projection of the OH0 bond on the a axis is 2.0 A and the absorption is strongest in the polarization parallel to this axis there is no doubt about the relation between the absorption and the B3-type OH stretching. Already indicated in the spectrum of the polycrystaihne samples, the separation of the broad absorption into two main regions is clearly apparent in the spectra of the oriented samples of both HPA and HPA,. The minimum of absorption is, in both, near 1240 cm-‘. However, the lower frequency part of the absorption is stronger in HPA and extends further down whereas in HPA, this part, is narrower and the one above 1200 cm-’ more pronounced so that the center of gravity of the absorption seems to be shifted on deuteration to higher frequencies. This anomalous deuteration effect has to be considered together with others described in previous paragraphs, i.e. the appearance in the spectra of HPA, of surplus PH:! deformation and OPO stretching bands, and of t.he oOH0 band. The Iatter should not be active in the Ds factor group. Considering the number of lattice translations (T’) and acoustic modes (T) the aOH fundamental of B, symmetry is redundant. In fact, it was not found in the spectrum of HPA. The appearance of this and of the other bands mentioned which are not allowed in this factor group clearly indicate a Iowering of the effective symmetry of the H2P01 group. Since the X-ray precession photographs of HPA and HPA, are identical it may be assumed that the lowering of the local symmetry is associated with the increase of the number of molecules in the unit cell which would allow for the appearance of more bands and, in particular, would activate the oOH0 mode in the IR spectrum. It is very likely that deuteration is accompanied by an increase in the 0 - - - 0 distance. Now, the positive geometrical isotope effect has been explained in terms of the interaction between the OH and

he GOHO &etchings in double-well potentials [ll].There are no indications If proton tunnelling in the sense of level splittings in the spectrum of HPA, butfor HPA, an asymmetric potential is likely in view of the above arguments. ‘he change of symmetry results obviously from an interplay of the proton tith the lattice vibrations including the hydrogen bond stretching. Comparison of the spectra Ila containing the &-type component of the )H stretching with the spectra of the polycrystalline samples show definite .ifferences. Thus in the spectra of HPA the minimum of absorption -1240 cm-’ i much more pronounced in the polarized spectrum and also the absorption Inthe higher frequency side of the minimum is stronger. The difference must le due to the BZ component which does not appear in the absorption of the c face, but occurs, superposed on the Bj component, in the polycrystalline naterial. Considering the minimum of absorption near 1240 cm-’ the uestion arises whether there are in fact two vOH bands or is the minimum esulting from an Evans type “window” dividing the broad absorption into wo parts. Usually strong and broad absorption minima in the ZJOHband are aused by overtones or combinations involving other OH modes since for uch minima a strong interaction between the broad fundamental and the lvertone is required 1121. However, no protonic mode seems to be involved ince the “window” does not move with deuteration. Moreover, there is no Itherobvious candidate for the Fermi resonance that might cause the window”. On the other hand, two components of the vOH absorption which re separated by several hundred wavenumbers have been observed in the bolarizedspectra of some acid salts of carboxylic acids 1131. The mechanism ;ivingrise to such large splittings is, however, not the simple proton tunnelling butprobably the interaction of strongly polarizable hydrogen bonds in the :rystals (141 or phonon mediated couplings between hydrogen bond ibration. UZKNOWLEDGEMENTS

This work was supported by the Boris Kidrilr Fund. Thanks are due to ‘ref. Golie for the X-ray facilities, to Mr. R. Gunde for recording the Raman pectra and to Mr. F. Cvek for the skilful technical assistance. tEFERENCES 1 2 3 4 5

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288 10 k Novak, Struct. Bonding (Berlin), 18.(1974) 177. 11 T. k Singh and 3. L. Wood, J. Chem. Phys., 50 (1969) 3572. 12 S. Bratos, J. Chem. Phys., 63 (1975) 3499. 13 D. HadZi and’ B. Orel, to be published.

14 R. Janoschek, E. G. Weidemann and G. Zundel, J. Chem. Sot., Faraday Trans. 2, 69 (1973) 505.