Polarized IR-microscope spectra of guanidinium hydrogenselenate single crystal

Spectrochimica Acta Part A 61 (2005) 2953–2965

Polarized IR-microscope spectra of guanidinium hydrogenselenate single crystal M. Drozd∗ , J. Baran Institute of Low Temperature and Structure Research of the Polish Academy of Sciences, Ok´olna 2 str., 50-422 Wrocław, Poland Received 17 September 2004; received in revised form 5 November 2004; accepted 8 November 2004

Abstract The polarized IR-microscope spectra of C(NH2 )3 ·HSeO4 small single crystal samples were measured at room temperature. The spectra are discussed with the framework of oriented gas model approximation and group theory. The stretching νOH vibration of the hydrogen bond ˚ gives characteristic broad AB-type absorption in the IR spectra. The changes of intensity of the AB with the O · · · O distance of 2.616 A bands in function of polarizer angle are described. Detailed assignment for bands derived from stretching and bending modes of selenate anions and guanidinium cations were performed. The observed intensities of these bands in polarized infrared spectra were correlated with theoretical calculation of directional cosines of selected transition dipole moments for investigated crystal. The vibrational studies seem to be helpful in understanding of physical and chemical properties of described compound and also in design of new complexes with exactly defined behaviors. © 2004 Elsevier B.V. All rights reserved. Keywords: Guanidinium selenate; Polarized IR; IR-microscope; Single crystal; Hydrogen bonds

1. Introduction The most important role in structure of new compounds is reserved for weak intermolecular interactions such as hydrogen bonds [1,2]. A vibrational spectroscopy is a very good tool for study of the behavior of hydrogen bonds in a crystal. The influences of the hydrogen bond network on the optical properties of the crystals were already discussed [3]. Crucial role in this study can be reserved for vibrational polarized light spectra on monocrystal’s samples. These results could be compared directly with theoretical calculation of optical properties [4]. The new family of compounds with guanidinium cation were discovered and investigated as potential materials for non-linear optics (NLO). Molecules with symmetry close to three-fold rotational (octupolar molecules) can exhibit nonzero ␤, despite being non-polar [5]. A number of molecules as well as molecular ions of D3h (or C3 or D3 ) symmetry have been shown to display promising properties. ∗

Corresponding author. Tel.: +48 71 3435021; fax: +48 71 3441029. E-mail address: [email protected] (M. Drozd).

1386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2004.11.007

The choice of guanidinium ion C(NH2 )3 + for detailed investigations is not accidental. The guanidinium ion can form a broad family of hydrogen-bonded crystals, but this cation is relatively simple chemical species [6] whose structure is related to those of amides and proteins in which there is considerable current interest. Some of these organic molecules have the delocalized electron systems and are of particular interest in NLO investigation because of their potentially large non-linear optical response [7]. The crystal structure and powder vibrational spectra of C(NH2 )3 ·HSeO4 at various temperatures were investigated previously [8]. The studies did not explain all chemical and physical behaviors of described compounds. The appearance of a phase transition seems to be not clear. If this phenomenon has to do with the many hydrogen bonds in the crystal, polarized IR spectra may provide a satisfactorily answer. This crystal belongs to centrosymmetric space group (P21 /n), but detailed vibrational studies of chemical interactions between organic (guanidinium ion) and inorganic (hydrogenselenate ion) parts can be helpful in work on next guanidinium cation family compounds. The results obtained during analysis of polarized vibrational spectra for investigated crystal could be

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used to discuss a design strategy for the molecular engineering for new crystals with similar crystallographic structure. The second problem which should be explained after detailed vibrational study, concern the “rigidity” of guanidinium cation (the shape of this cation is not deformed by chemical interaction, practically). This configuration is very stable and the symmetry is virtually D3h . The ion is almost flat. This property is independent from neighborhood of different chemical anions and interactions. In this configuration planarity of the CN3 groups skeleton correspond to sp2 hybridization of the carbon atom. The understanding of this behavior is very important in design of new compounds without inverse center, which will be interesting in technological applications.

2. Experimental The crystals of C(NH2 )3 ·HSeO4 were obtained by a slow evaporation at constant temperature (20 ◦ C) of the water solution containing guanidine cations (guanidinium carbonate) and selenate anions (selenic acid) in stoichiometric ratio 1:1. The transparent single crystals were very small (size 3 mm × 2 mm × 1 mm approx.) therefore were not suitable for standard IR and Raman polarized measurement. The polarized IR-microscope studies for investigated crystal with this small size were possible only. The attempt to growth of deuterated analogue was not successful. The crystals were oriented using X-ray and polarizing microscope methods. Two samples were prepared. One sample was parallel to b(Z) axis. The second one, the ac(XY), was perpendicular to the b(Z) axis. The polarized reflection spectra from (0 1 0) plane using Specular Reflectance Mode of IRmicroscope were acquired at every 10◦ with respect to the X axis. The relationship between X, Y principal optical axis and crystallographic a and c axis are shown in Fig. 2. The IRmicroscope polarized single crystal spectra of ac(XY) plane were recorded for one sample with different setting and fixed polarizer. For each specular spectrum new reference set was measurement. The reflectance spectra from a polished sample perpendicular to (0 1 0) face, with polarization of the radiation parallel to b(Z) crystallographic axis was recorded too. All polarized IR-microscope spectra were measured at room temperature. The sample was fixed on the microscope stage in air atmosphere. The auto moving function of microscope stage was switched off. The specular reflectance spectra were run on Perkin-Elmer AutoIMAGE IR-microscope, attached to a Spectrum 2000 FT-IR spectrometer with LN2 cooled MCT detector (spectral region 4000–600 cm−1 , resolution 4 cm−1 , number of scans 500, aperture 100 ␮m × 100 ␮m). The auto-focus function was applied to find and store information regarding the position across the monocrystal was. The standard build-in controllable polarizer was used. The absorbance spectra were calculated from specular reflectance spectra by Kramers–Kronig transforma-

tion module from Perkin-Elmer Spectrum version 2.00 software. The powder infrared spectra were measured using a Bruker IFS-88 spectrometer with resolution of 2 cm−1 . The samples in Nujol suspensions were used. A mini color CCD camera attached to the IR-microscope recorded photographs of the surface of the measured crystals.

3. The crystal structure and vibrational selection rules The C(NH2 )3 ·HSeO4 crystallises in P21 /n space group of the monoclinic system. There are four formula units per primitive unit cell [8]. The crystal is build up of hydrogenselenate anions that are connected by hydrogen bonds forming infinite chains (Fig. 1a). These chains are parallel to the b crystallographic axis and form planes parallel to the (0 1 1) crystallographic plane. The hydrogen bonds linking the hydrogenselenate anions into chains are medium strong with ˚ The O2 O(2) H(1) · · · O(1)#1 distance equals 2.616(6) A. oxygen atom plays a role of proton donor, whereas the O1 oxygen atom is an acceptor of proton in hydrogen bond. The Se O2 bond is the longest in hydrogenselenate anion. The Se O1 bond is the shortest one. The second type of crystal sublattice is built up of guanidinium cations C(NH2 )3 + (Fig. 1b). The cation has symmetry similar to D3h , but this relationship is disturbed by second hydrogen bond network, because all oxygen atoms from hydrogenselenate anions participate in other hydrogen bonds joining the HSeO4 − ions with nitrogen atoms from guanidinium cations. The bonds are very weak with distances: ˚ N(1) H(12) · · · O(3) = 2.902 A, N(1) H(11) · · · O(4) = ˚ N(2) H(22) · · · ˚ N(2) H(21) · · · O(4) = 2.923 A, 2.989 A, ˚ ˚ and O(2) = 3.082 A, N(3) H(31) · · · O(3) = 2.910 A ˚ The weak hydrogen bonds N(3) H(32) · · · O(1) = 2.983 A. change C N distances in guanidine ion and investigated cation does not have D3h symmetry exactly. The C N distances are different and equal to: 1.287 (C N(1)), 1.307 ˚ The differences were (C N(2)) and 1.335 (C N(3)) A. found in the N H distances in guanidinium ion. The bonds ˚ N(1) H(11) and N(1) H(12) are equal to 0.829 and 0.558 A, respectively. The distances N(2) H(21) and N(2) H(22) ˚ whereas the are similar and equal: 0.694 and 0.684 A, bonds N(3) H(31) and N(3) H(32) are equal to 0.750 and ˚ respectively. The guanidinium ion is almost flat, 0.778 A, but the very small (ca. 3◦ ) deviation of hydrogen atoms from C N plane is observed. The projection on ac plane of guanidinium selenate structure is presented in Fig. 2 (above: the hydrogenselenate hydrogen bond network; below: the second motif of crystallographic structure—guanidinium cations). The X-ray diffraction study of the crystal structure reveals that the hydrogenselenate anions (deformed tetrahedrons) and the guanidinium cations occupy the C1 positions in the primitive unit cell.

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modes for the internal vibrations of the guanidinium cation, 9Ag + 9Bg + 9Au + 9Bu modes for the internal vibrations of hydrogen selenate ion, 3Ag + 3Bg + 3Au + 3Bu active modes of hydrogen bond vibrations, 6Ag + 6Bg + 5Au + 4Bu translational modes and 6Ag + 6Bg + 6Au + 6Bu librational lattice modes. Each internal vibrations A1 and A1 of the guanidinium anion and non-degenerate hydrogenselenate anions ν1 vibrations should be split into four components being allowed either in IR (Au + Bu ) or in Raman (Ag + Bg ) spectra, respectively. For the ν2 (E), ν3 (F2 ) and ν4 (F2 ) vibrations of selenate anion and E and E internal vibration of guanidinium cation the double and triple degeneration is removed and each component is split into Ag , Bg , Au and Bu modes due to the factor group coupling.

4. Results and discussion The polarized IR-microscope spectra of C(NH2 )3 ·HSeO4 single crystal measured at room temperature are shown in Figs. 3 and 4. The wavenumbers of the bands observed in the IR and their assignments are presented in Table 2. Fig. 5 shows the changes of intensity in polarized FT-IR spectra of described single crystal for sample (0 1 0) measured for various orientation of the electric vector. The bands observed in the 4000–600 cm−1 region in the measured spectra are due to hydrogen bonds, internal vibrations of guanidinium cations and internal vibrations of hydrogenselenate anions. The photographs of surfaces of investigated crystal are shown in Fig. 6. For guanidinium selenate crystal the three surfaces for various positions of samples. The surfaces of small crystals are very rough. The IR-microscope offers the possibility to choose the best suitable area of the crystal as regards defects and/or contamination. The black squares represent clippings of surfaces of crystal where the microscope infrared spectra were measured. The size of this surface could be modified by IR-microscope aperture function.

Fig. 1. (a) The projection of guanidinium hydrogenselenate crystal structure (only the hydrogenselenate ions) onto the bc (1 0 0) plane; (b) the projection of guanidinium selenate crystal structure (only the guanidinium cations) onto the bc (1 0 0) plane.

4.1. The hydrogen bond vibrations 4.1.1. The νOH stretching vibrations The stretching type of vibration of hydrogen bonds νOH displays a well defined and medium strong absorption band in the IR spectrum with the AB structure occurring in the region above 1500 cm−1 . This broad absorption is the most

The fundamental modes analysis for the described crystal is given in Table 1 (for guanidinium ion the D3h symmetry approximation was used). The formal classification of fundamental modes gives 24Ag + 24Bg + 24Au + 24Bu Table 1 Fundamental vibrational analysis for guanidinium hydrogenselenate Internal SeO4 2−

Lattice mode

Ag Bg Au Bu

Guanidinium internal modes

Hydrogen bond

Spectral activity

A

T

L

ν1

ν2

ν3

ν4

A1

A1

E

E

νOH

δOH

γOH

1 2

6 6 5 4

6 6 6 6

1 1 1 1

2 2 2 2

3 3 3 3

3 3 3 3

4 4 4 4

4 4 4 4

8 8 8 8

8 8 8 8

1 1 1 1

1 1 1 1

1 1 1 1

A: acoustic, L: librational, T: translational, R: Raman activity, IR: infrared activity.

R R IR (c) IR (a, b)

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Fig. 2. The projection of guanidinium hydrogenselenate crystal structure (only the hydrogenselenate ions—above; the guanidinium cations—below) onto the ac (0 1 0) plane. a and c denote the crystallographic axes. X and Y denote the principal directions for the visible light. The circle shows the orientation of the electric vector for the polarized IR measurements.

characteristic feature of the IR spectrum. It seems to be clear that broad band with two maxima A and B arise from the stretching vibrations of the medium hydrogen bond ˚ The AB structure of νOH with O · · · O distance 2.616 A. mode observed in IR spectrum of guanidinium selenate could be explained by Fermi resonance between the νOH fundamental and overtone of the δOH mode [9]. The second way to describe the characteristic local AB maxima in the solid state is proposed by theory of anharmonic coupling between the high-frequency stretching vibration νOH and low frequency lattice phonons [10]. For other hydrogen bonds observed in the titled crystal (hydrogen bonds joining

the nitrogen atoms of guanidinium cation with oxygen atoms of hydrogenselenate ions) the broad absorptions and Fermi resonance between νNH and δNH is not observed. Such broad absorptions was observed in compound with shorter NH · · · O bonds especially in complexes with intramolecular NH · · · O chemical bonds [11], but in such a case the NH · · · O distances are very short. As follow from X-ray structure (Fig. 2) and theoretical values of the square direction cosines of transition dipole moments (TDM) (Table 3) for O · · · O bond the νOH of the O2 H1 · · · O1 hydrogen bond should give absorption in the spectra polarized close to the Y axis, assuming that the

M. Drozd, J. Baran / Spectrochimica Acta Part A 61 (2005) 2953–2965

transition dipole moment of the νOH mode is parallel either to the O · · · O or to the O H direction. According to this prediction the νOH mode should give rise to a broad absorption observed in the spectra of the 0 1 0 (ac) sample (Fig. 4). In this absorption one can determine the following maxima: at 2789 and 2448 cm−1 . Unexpectedly the A and B bands are observed in all spectra measured on 0 1 0 surface sample and the intensity is changeable. In the spectrum measured

2957

(Fig. 3) at 0◦ position of polarizer the intensity of A and B maximum is very small. The intensity of the A and B bands depends strongly on the polarization. The strongest intensity is observed in spectra measured for 60, 70, 80, 90, 100 and 110◦ orientation of the electric vector. These results seem to be to some extent in agreement with theoretical calculation of TDM because in this approach the band should have the strongest intensity in c and small intensity in a directions

Fig. 3. The polarized FT-IR spectra of guanidinium hydrogenselenate single crystal XY (0 1 0) sample measured for various orientation of the electric vector.

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Fig. 3. (Continued ).

(square directional cosines ratio c/a ∼ = 2.6 for νOH (O H)), whereas in b direction the band νOH is not visible, practically. The measured intensity ratio is a little different. One cannot expect the better agreements if the oriented gas model approximation is used. The wavenumbers of the A and B bands observed in spectra measured for the ac sample (perpendicular to b optical axis) are dependent on polarization. Particularly the wavenumbers of the A band show big differences. Its value changes between 2763 cm−1 (at 10◦ ) and 2888 cm−1 (in spectrum measured at 70◦ ). The changes are not linear and are not correlated with the orientation of electric vector. Some

similar changes of frequencies were observed for B band. This B sub maximum is recorded in wide range (2418 and 2470 cm−1 ) of infrared spectrum. These wavenumbers fluctuations are independent on the orientation of electric vector. The biggest differences are observed between spectra measured along the b-axis and perpendicular to it. It seems that a little higher wavenumbers of bands A and B are recorded in spectra measured for electric vector oriented in range 60–120◦ , but this correlation is very poor. These differences in observed frequencies origin from the strong coupling between the OH stretching vibrations of the adjacent hydrogen bonds in the same chain. As they are related by the two-fold

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2959

Fig. 4. The polarized FT-IR spectra of guanidinium hydrogenselenate single crystal measured for various orientation of the electric vector.

screw operation, therefore, their in-phase coupling gives the lower frequency band polarized along b-axis, whereas the out-of-phase coupling gives the higher frequency band polarized perpendicular to the monoclinic b-axis. Similar situation was observed for CsHSO4 crystal in reflection spectra measured on monocrystal’s samples [12]. 4.1.2. The δOH and γOH hydrogen bond deformation vibrations The deformation vibrations of these hydrogen bonds are expected in the 1270–800 cm−1 range [13]. The proper assignment for the in-plane (δOH) bending vibrations is difficult. In some cases this mode was observed at low temperature powder infrared spectra [14], only. According to calculations of TDM (see Table 3) this band should be observed in the IR spectrum polarized to the a crystallographic axis. The medium broad band at 1262 cm−1 appears in this spectrum, however in the spectra of ac face the other four bands were observed. These bands are medium and broad. The two bands are observed at 1294 and 1226 cm−1 in spectrum measured at 10◦ (X) angle of polarized light and two bands at 1291 and 1248 cm−1 very clearly appear in spectrum for 100◦ (Y) position of polarizer. The intensity seems to be too strong as one

should expect for pure δOH vibrations. On the basis of theoretical calculation the broad band observed at 1226 cm−1 in spectrum polarized parallel to the X axis was assigned as derive from in-plane deformation vibration of hydrogen bond. The out-of-plane bending modes γOH could be identified in powder and in polarized spectra. This band is observed at 808 cm−1 in the powder spectrum and appears as shoulder at 810 cm−1 in the spectrum polarized parallel to the b(Z) axis and also in region 845–800 cm−1 in some spectra recorded for the (0 1 0) sample. According to theoretical calculation it should be active in polarized spectra measured almost parallel to b crystallographic axis. This experimental value corresponds very poor to the predicted orientation of the transition dipole moments (see Table 3). This band should be assigned to out-of-plane bending modes of hydrogen bond. 4.1.3. The internal vibration of the hydrogenselenate ion The stretching vibrations of the Se O bonds appear in the region between 1000 and 800 cm−1 . They originate from ν1 SeO4 3− (A1 , 837 cm−1 ) and ν3 SeO4 3− (F2 , 876 cm−1 ) of the free SeO4 3− anion. The bending vibrations of the free selenate anion with Td symmetry are described as ν4 SeO4 3− (F2 , 415 cm−1 ) and ν2 SeO4 3− (E, 345 cm−1 ). The wavenumbers of the vibration of “free” ion are taken from Ref. [15].

2960

Table 2 Observed frequencies and tentative assignment for guanidinium hydrogenselenate crystal ac plane 10(E X)

0

20

30

40

3796w

3796w

3798w

3796w

3796w

3730w

3732w

3732w

3731w

3731w

50 3794w 3745w

60 3796w

3686w 3667w 3644w

3644w

3666w

80

90

3798w

3730w 3707w

3667w 3644w

70

3730w

3797w 3733w

3706w 3669w 3643w

100(E Y)

110

120

140

150

160

170

3796w

3797w

3796w

3796w

3797m

3731w

3730w

3731w

3731m

3669w 3642w

3369m

3795w

3795w

3796w

3796w

3730w

3729w

3731w

3731w

180

E Z(b)

130

Powder sample

Overtone Overtone Overtone Overtone Overtone

3705w 3667w

3670w

3667w

3667w 3644w

Tentative assignment

3626vw 3606w 3584w 3555w

3558m

3441s

3447s

3491w 3440s

3414sh

3448s

3453s

3454m

3453m

3487w 3439m

3474sh 3441m

3421sh

3418sh

2425sh

3411sh

3396sh

3402w

3436m

3437m

3383w 3378m

3326m

3287w 3254m

3327m

3265m

3380sh

3337w 3319w

3266w

3382m

3331m

3289w 3266w

3336w

3211w

3362w

3338w

3343w

3342w 3325w

3283w

3302w 3272w

3432s

3434vs

3435vs

3440vs

3440vs

3384m

3447vs

3354m 3344w

3269w

3341w

3267w

3338m

3268w

3336m

3448vs

3449vs

3415sh

3419vs

3378m

3349s

3329m

3331m

3333m

3329m

3331m

Sym. NH2 str

3263m

3261m

3263m

3263m

3262m

3268m

3210m

3213m

3208m

3198m

3169m

3171m

3172m

3337vs

Sym. NH2 str

3262vs

Sym. NH2 str

3270m

3255w 3214w

3210w

3216vw

3220vw

3212w

3205vw

3203vw 3184w

3177w

3212vs 3207m

3184w

3182w

3172w

3170w

3170m

3175vs

3100vw

2763vw

2955s 2924s 2868s 2854m

2788vw

2756w

2751w

2773w

2788w

2781w

2791vw

2789w

2801w

2805vw

2728vw

2443vw 2375w 2340w

2425w

2418w

2441vw

2443w

2471w 2446w 2375w 2345w

2470w 2375w 2345vw

2461w 2378w 2348vw

2460w 2370w 2339w

2462vw 2448w 2371w 2339w

2469vw

2723m 2678m

2445w 2364w 2339w

2441m

2465w 2449vw

2376w

2746vw

2151sh 1987w 1864vw

1849vw 1824vw

1784w 1768vw 1729w

Sym. NH2 str Sym. NH2 str

3167w

2928vw

2434vw 2371w 2343vw

Asym. NH2 str Asym. NH2 str Asym. NH2 str

3340m

3263w

3186w 3167w

3380w

3435s

3260w

3217w 3211m

3357w

3285w

3251w 3204m

3375w 3355w

3434s

1765vw 1731w 1712w

1698sh

1727w 1697sh

1768vw 1728vw

1787w 1768vw

1789w 1767vw 1693w

1785w 1765vw

1771vw 1734vw

νOH B νOH B Overtone Overtone Overtone

M. Drozd, J. Baran / Spectrochimica Acta Part A 61 (2005) 2953–2965

3515vw

1656s 1592w

1660s 1634sh 1615w

1665s 1635m 1600w 1553w

1668m 1640w 1592w 1564w

1681m 1639m

1681m 1638m

1671vw 1638m

1679w 1638m

1670m 1637m

1667m 1634m

1666s 1635m

1662s 1639m

1660s 1636m

1660vs 1633w

1659vs 1636m

1565w

1565w

1566m

1567m

1566m

1566m

1566m

1568w

1570w

1569w

1570w 1556w 1537w

1555vw 1535w

1536vw

1536vw

1538vw

1540vw

1538w

1538w

1537w

1537w

1537w

1503w

1504w

1504w

1452vw

1454w

1455w

1505vw

1659vs 1634m 1613w 1572w 1554w 1536w

1660vs 1634w

1661vs 1633w

1664vs 1637sh

1555w 1538vw

1571w 1554w 1537vw

1556w 1538w

1504vw

1504w

1503w

1505vw

1452vw

1452vw

1455vw 1414vw

1416vw

1646vs 1564s 1555m 1539m 1535m

1505w 1453vw

νas CN νas CN δNH2 δNH2 δNH2 δNH2 δNH2

1466w 1457vw 1377vw 1365vw

1292w

1294w

1338vw 1293w

1295w

1296w

1296w

1296w

1332vw 1295w

1296w 1247w

1291w 1249w

1291w 1248w

1292vw 1251w

1291vw 1251vw

1249w

1251w

1293w

1251w

1262m

1292w 1262m

1226w

CN3 angle δOH

1211w

956vs

874w

958vs 923w 914sh 877w

958vs

958vs

961s

963s 944sh

959s 946m

951sh 934s

967m 931s

970m 925vs

966m 924vs

966m 925vs

961m 926vs

960m 927vs

958s 930s

958vs 935s

959vs 937m

959vs 928m

959vs 918m

877w

876w

877w

879w

877m 834w

876m 834w

877m

876m

875m 831w

875m

875s

873s

874m

872m

876m

873w

875w

734vw

752vw 732vw

942s

836s 810sh

1098vw 1065vw 1008vw 964vs 942vs 909vs 871vs 833s

ρNH2 ρNH2 νs CN ν3 SeO4 ν3 SeO4 ν3 SeO4 ν3 SeO4 ν1 SeO4 γOH

771vw 752w

763w

760vw 731vw

756w

748w

745w

753w 733vw

731vw

714vw

726w

720w

714w

731s

716w

716w

712vs

711s 573m 531s 523s

CN3 def. inplane νSeOH

M. Drozd, J. Baran / Spectrochimica Acta Part A 61 (2005) 2953–2965

1556vw 1538vw

1673m 1638w

2961

2962

M. Drozd, J. Baran / Spectrochimica Acta Part A 61 (2005) 2953–2965

Fig. 5. The polarized FT-IR spectra of guanidinium hydrogenselenate single crystal XY (0 1 0) sample measured for various orientation of the electric vector (the changes of intensity for selected bands-details in text).

Unfortunately, the bands corresponding to the bending vibrations of hydrogenselenate ions could not be observed in IR-microscope spectra because the MCT detector cooled by liquid nitrogen was used and spectra were recorder in the spectral range 4000–600 cm−1 , only. All modes are active in the Raman spectra and only the F2 species are allowed in the infrared spectrum. According to the X-ray data [8] the HSeO4 − ions occupy the site of C1 symmetry. It is characteristic that all modes arising from the internal vibrations of the hydrogenselenate anion are split into a few components in the infrared spectra. As the SeO4 3− anions occupy C1 site, a splitting is expected for the ν3 SeO4 3− and for ν4 SeO4 3− triple degenerated modes (3Au + 3Bu —infrared spectrum). The splitting for the non-degenerate ν1 SeO4 3− (Au + Bu —infrared spectrum) and for double degenerate ν2 SeO4 3− (2Au + 2Bu —infrared spectrum) modes should be observed. According to the fundamental vibrational analysis the Au symmetry modes of ν3 vibrations should be active in the IR spectra measured parallel to the b crystallographic axis, whereas the modes Bu should be polarized perpendicularly to the b axis. In the spectrum polarized parallel to the Y direction two bands are observed at 966 and 924 cm−1 in region of ν3 vibrations. The band at 966 cm−1 has a strong intensity, whereas the second one (924 cm−1 ) is the strongest in the measured region. The band observed at 966 cm−1 is asymmetric with broad basis. It is probable that at ca. 985 cm−1 we can observe very broad shoulder.

In the spectrum polarized at E X one band at 958 cm−1 is observed. This band is the strongest one in this IR spectrum. The assumed splitting of this band resulting from fundamental vibrational calculations is not observed. The band at 958 cm−1 is very narrow but seems to be an asymmetric. It is very characteristic that changes in shape, frequencies and splitting of this band are observed in polarized spectra measured on the ac sample. In the spectra measured at the range 0–40◦ one band at 956 cm−1 and one shoulder appears, but decreasing of intensity for described band is observed. From 20◦ angle position of polarizer the shoulder is not observed. In the IR spectrum recorded at 50◦ the splitting into two components is seen, but with the opposite ratio of intensities than in spectrum measured close to the E Y (E 60). The intensity of the band at 959 cm−1 is bigger than observed for the band at 946 cm−1 . In next few spectra of these series (ac sample) the intensity of the band at 973 cm−1 is smaller then that at 935 cm−1 . Next changes in intensity ratio are visible in the spectrum measured at 140◦ position of polarizer. In this spectrum the intensity of two bands is equal. Next spectra should be identical as measured in the range 0–40◦ of the polarizer angle. According to Table 1 the band originating from ν3 vibrations of hydrogenselenate anion should be active in polarized spectrum recorded parallel to the Z(b) direction. This expected band appears at 942 cm−1 but this one has “only” a strong intensity (the band is not the strongest in the whole measured spectrum). The single band is symmetrical without expected dynamical splitting.

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(the intensity seems is a little bigger in the spectrum measured parallel to Y direction). This band is visible in polarized spectrum measured parallel to Z(b) axis at 836 cm−1 , but intensity of this band is strong. Very interesting phenomenon is observed in this spectrum at 712 cm−1 in the polarized spectrum measured parallel to Z(b) axis. The intensity of this band is strong. This band is very sensitive to changes of the electric vector. In the spectra measured on ac sample this band is invisible. In this region of the infrared spectra two weak bands are recorded in the spectra measured at 90◦ , 110◦ and 120◦ position of polarizer. In the theoretical approach of transition dipole moment (TDM) for hydrogenselenate anion (Table 3) one bond should give big absorption in the spectrum parallel to Z(b) axis. The ˚ The oxylength of this Se(1) O(2) bond is equal to 1.712 A. gen O(2) atom plays a role of proton donors and this bond is the longest among selenium–oxygen bonds which were de˚ The scribed in the hydrogenselenate ions (1.605–1.620 A). difference in length of Se O bond should be visible in the infrared spectrum. According to this theoretical calculation the band at 712 cm−1 recorded in polarized spectrum measured parallel to Z(b) crystallographic axis was assigned to νSeOH vibration. This exceptional agreement between theoretical approach (very simple oriented gas model was used) and experimental data should be emphasized.

Fig. 6. Photographs of surfaces of investigated crystals. For each crystal three surfaces for various positions of samples were photographed (see details in text).

As originating from ν1 vibration of SeO4 3− anions was assignment the band at 834 cm−1 in the spectra measured at 60◦ and 70◦ . This band is weak. The frequency is practically constant. In polarized spectrum measured at 100◦ of the polarizer position this band appears at 831 cm−1 . In polarized spectra measured on ac sample its intensity is not changed Table 3 Square directional cosines of selected transition dipole moments for guanidinium hydrogenselenate crystal a

b(Z)

c* ⊥ a

O2 H1 · · · O1 hydrogen bond νOH ( O H) γOH (⊥ O H · · · O) δOH (⊥ O H) νOH ( O · · · O) γOH (⊥ Se1 O2 H1) δOH (⊥ O · · · H)

0.2480 0.0013 0.7506 0.0072 0.0015 0.9529

0.0117 0.9878 0.0004 0.0114 0.9876 0.0002

0.7404 0.0108 0.2489 0.9816 0.0107 0.0469

Hydrogenselenate anion Se1 O1 Se1 O2 (donor) Se1 O3 Se1 O4

0.5035 0.0001 0.8078 0.0580

0.1231 0.9850 0.1168 0.0140

0.3734 0.0146 0.0750 0.9283

4.1.4. The guanidinium cations vibrations The discussion of the directional properties of absorption connected with particular bonds in the guanidinium cations seems to be very difficult. The symmetry of guanidinium cations is virtually (close to) D3h . The hydrogen bonds joining hydrogen atoms of guanidinium ions with oxygen atoms of hydrogenselenate anions are very weak and deformation of the original symmetry is relatively small. In measured IR spectra some differences in intensities of the bands derived from guanidinium cations should be observed. The fundamental vibrational analysis for the guanidinium cations is presented in Table 1. The guanidinium internal modes should be split into few components: four bands for non-degenerate A1 and A1 modes and eight bands for double degenerate E and E modes are expected. The internal vibrations of the guanidinium cations may by analyzed taking into account the vibrations of two functional groups: NH2 and CN. The stretching and bending vibrations of the NH2 groups may be considered as vibrations of the N H · · · O hydrogen bonds. For such N H · · · O hydrogen bonds one may expect the νNH bands close to 3100 cm−1 according to Nakamoto correlation [16]. As the N H · · · O hydrogen bonds in the guanidinium sele˚ ˚ the vibranate crystal are similar (2.902(8) A–3.082(8) A), tions of NH2 group may be considered as antisymmetric (νas NH2 ) and symmetric (νs NH2 ) stretching type of the coupling with the corresponding bands observed at 3400 and ca. 3300 cm−1 , respectively. The other low frequency bands (till ca. 2000 cm−1 ) are typical for the N—H · · · O

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hydrogen bonds and should be assignment as arising from overtones [17]. The very strong and sharp band that appears at 3447 cm−1 in polarized spectrum measured parallel to the X axis is assigned to (νas NH2 ). This band appears in the polarized spectra measurement at 3448 cm−1 (E Z(b)) and at 3452 cm−1 (E Y). It seems that band at 3452 cm−1 should originate from N(2) H(22) bond, where the H(22) proton is involved in the weaker hydrogen bond ˚ The bands N(2) H(22) · · · O(4)#3 with distance 3.082(8) A. appear at 3327, 3211 and 3167 cm−1 in the spectrum measured at E X axis and at 3349, 3268 and 3198 cm−1 in the spectrum measured for Z(b) direction. These bands are observed at 3341, 3267, 3203 and 3177 cm−1 in the spectrum measured at E parallel to Y. The intensity of these bands is smaller than bands originating from the νas NH2 . According to these observations the described bands were assigned to the νa NH2 . The deformation vibrations of the NH2 group should be observed in the region between 1650–1550 cm−1 (in-plane deformation) and in the region 1150–1100 cm−1 (rocking type). In first region of the polarized spectrum measured parallel to the X direction three bands were observed at 1592, 1535 and 1503 cm−1 . Counterparts of these bands are observed at 1556, 1538 and 1505 cm−1 in the spectrum recorded at E Z(b) and at 1566, 1538 and 1508 cm−1 in polarized spectrum parallel to Y axis. These medium and weak bands were assignment to δNH2 . The recorded frequencies of these bands are in good agreement with bands observed in spectra of other guanidinium complexes [6]. The problem with bands originating from out-of-plane vibrations of the NH2 group is noticed. According to earlier data [8] the band at 1098 cm−1 in the powder spectrum was assigned to ρNH2 . This band had a weak intensity in this spectrum and is not observed in polarized spectra. The bands observed in polarized spectrum (E X) at 1660 cm−1 was assigned to νas CN. In polarized spectrum measured parallel to the Y direction this band is observed at 1667 cm−1 . Surprisingly, the intensity of this band is very different in described spectra. In the spectrum (E X) the intensity of this band is very strong, bigger than intensity of bands derived from antisymmetric vibrations of NH2 group. Opposite situation is observed in the second spectrum (E Y). In this case the intensity of the band corresponding to νas CN vibration is smaller than intensity of the band derived from νas NH2 vibration. In polarized spectrum measured parallel to the Z(b) axis the band appears at 1664 cm−1 but its intensity is the biggest one in whole recorded IR spectrum. The changes in intensities of this band are difficult to explain on the basis of our experiment and in the framework of the theoretical calculations of TDM using oriented gas model, also. The very characteristic band originating from the symmetric CN stretching vibrations appears in powder IR spectra at 1008 cm−1 . The intensity of this band is very weak (in contrary in Raman spectrum the intensity is very strong). This band is absent in all components of measured IR-microscope spectra. The band appears at 731 cm−1 in the polarized spectrum measured at E Y having medium intensity was as-

signed as derived from the in-plane deformation of the CN3 group. In the spectra (E X and E Z(b)) this band was not detected. The NH2 twisting vibrations should appear [6] at ca. 830 cm−1 . The intensity of these bands is very weak and other bands which derive from ν1 SeO4 ion vibration in this region of the spectrum were recorded. The assignment of this band is very difficult. Unfortunately the observation of CN3 angle deformation (out-of-plane at ca. 500 cm−1 ) owing to the MCT detector used in the experiment is impossible.

5. Conclusions The IR-microscope polarized specular reflectance spectra were obtained for GuSe monocrystal sample at room temperature for the first time. Recording of IR spectra of a single crystal using polarized light was possible with an IRmicroscope only. It allows choosing of the best area (without defects and/or contamination) of the crystal for measurements. Preparing the single crystals (surface polishing and optical orientation) for good IR-microscope measurements is very difficult. The vibrational spectra seem to be in good agreement with the crystallographic data published previously. The hydrogenselenate anions and guanidinium cations in described crystal have the C1 symmetry. The polarization properties of the bands in the polarized IR spectra are discussed on the basis of the oriented gas model approach. Most bands of the internal vibrations of ions and vibrations of the hydrogen bonds have been identified. On the basis of theoretical approach of TDM the previous assignments of the observed bands in powder spectra were completed and improved. The infrared spectra are dominated by a characteristic broad absorption (of a doublet (AB) structure) arising from the stretching vibrations of the hydrogen bonds. The intensity of the B band is a little weaker than of the A band. The Davydov-type (correlation field or factor group) splitting is observed for the νOH modes. The polarized infrared spectra show that the stretching vibrations of the shortest hydrogen bonds give strong broad absorption polarized parallel to Y. The direction behavior of bending vibrations of this hydrogen bond is as expected as well. It is not possible to separate the bands corresponding to νNH vibrations because the all NH · · · O hydrogen bonds are polarized similarly. The application of oriented gas model in the case of many bonds with similar distances is improper. Unfortunately, the new information about mechanics of the observed heat flow anomaly is not detected. The structure of GuSe (hydrogenselenate chains and very stable guanidinium cations involved in network of hydrogen bonds) is very stable. The disordering of protons in the hydrogen bonds (this phenomenon is not observed by crystallographic X-ray study at room temperature) may be responsible for mechanism of phase transition, only.

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Acknowledgments We are grateful to Prof. J. van der Maas for all helping, patience and stimulated discussion during “long coffee break”. The work was supported by NATO Scientific Advanced Fellowship Program.

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