Crystal matrix and crystal double matrix spectroscopy

Journal of Molecular Structure 704 (2004) 71–78 www.elsevier.com/locate/molstruc

Crystal matrix and crystal double matrix spectroscopy H.D. Lutz* Universita¨t Siegen, FB 8/Anorganische Chemie I, Postfach 101240, Siegen D-57068, Germany Received 5 November 2003; accepted 2 December 2003 Available online 10 May 2004

Abstract Infrared and Raman spectra of matrix isolated ions in crystalline solids display the normal modes of these units with respect to the symmetry of the respective lattice sites. Hence, they give information on all structure changes of the incorporated entities (energetic distortion), the amount of intramolecular coupling of the respective stretching modes, and symmetry and potential at the host crystal lattice sites. In the case of neat compounds, this information is hidden by collective solid-state effects. Assignment of the modes of matrix isolated guest ions can be performed by Raman single crystal experiments. If two different matrix isolated entities in a crystalline matrix come into contact forming complexes (double matrix spectroscopy) symmetry and frequencies of the modes of the guest ions are additionally changed. In the case of solid hydrates, thus the influence of metal ions and anions on the strength of hydrogen bonds (cooperative, competitive, and synergetic effects) can be analysed in detail. Examples of crystal matrix and crystal double matrix spectroscopic experiments are presented on orthorhombic halates MðXO3 Þ2 ; monoclinic halate monohydrates MðXO3 Þ2 ·H2 O (M ¼ Sr, Ba, Pb and X ¼ Cl, Br, I), and likewise monoclinic kieserite-type compounds 22 2þ MXO4 ·H2 O (M ¼ Mn, Co, Ni, Zn, and X ¼ S, Se) with matrix-isolated XO2 guest ions and HDO guest molecules. 3 ; XO4 ; and M q 2004 Elsevier B.V. All rights reserved. Keywords: Crystal matrix spectroscopy; Double matrix spectroscopy; Isotope dilution technique; Distortion of matrix isolated guest ions; Potential at the hostcrystal lattice sites; Intramolecular coupling of XO stretching modes; Halates and halate monohydrates; Kieserite-type sulfates and selenates

1. Introduction In the case of molecules or ions isolated in a crystal matrix, as in the case of crystal matrix spectroscopy, only symmetry and static potential at the respective lattice sites of the crystal can influence allowance and frequencies of the normal modes of the matrix isolated guest molecules or ions without any disturbance and band broadening by coupling and collective solid state effects. Thus, infrared and Raman spectra of matrix isolated molecules or ions reflect symmetry and potential of the respective lattice site very precisely. There are several variants of crystal matrix spectroscopy: 1. Isotope dilution technique, i.e. substitution of an atom by 5– 10% of a heavier or lighter isotope, 2. Solid solution spectroscopy, i.e. studying matrix isolated molecules or ions on lattice sites of a crystalline matrix, and * Tel.: þ49-271-740-2936; fax: þ49-271-740-2555. E-mail address: [email protected] (H.D. Lutz). 0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2003.12.059

3. Double matrix spectroscopy, i.e. two or more different matrix isolated entities in a matrix come into contact with another forming complexes.

2. Isotope dilution technique Isotope dilution techniques have been very frequently used in hydrogen-bond research as the so-called H/D isotope dilution technique, in the case of which samples deuterated or protiated for a few percent are studied [1 – 5]. By using H/D isotope dilution techniques, the frequencies of the OD or OH stretching modes of deuterated or protiated molecules matrix isolated in an almost fully protiated or deuterated matrix, respectively, can be recorded. They display the strength of hydrogen bonds more precisely than each other techniques [4 – 5]. Thus, in the case of solid hydrates, both intramolecular coupling of the two OH vibrations of a hydrate H2O molecule and intermolecular coupling phenomena are omitted [1,4,5]. The same is true for other entities with more than one hydrogen atom as NH3, 2 NHþ 4 , NH2 , etc. [5,6]. Also in the case of entities with only

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H.D. Lutz / Journal of Molecular Structure 704 (2004) 71–78

one hydrogen atom, as OH2 and SH2 ions, H/D isotope dilution techniques are indispensable because in solid hydroxides, sulfides, etc. unit cell group (Davidov, correlation, factor group) splitting effects strongly influence the frequencies of the observed infrared and Raman bands [5,7]. For isotopes of other atoms than hydrogen this technique is much less important because the much smaller differences of the respective masses. Thus, even in the case of solidstate lithium compounds, special bands being due to 6Li atoms and to 7Li atoms are not observed. In the older literature, bands caused by unit cell group splitting were interpreted by some authors as due to isotopic splitting. Isotopic splitting of normal modes, however, can only be observed in the case of matrix isolated molecules or ions due to atoms unlike those of the host compound because in this case collective solid state effects (as described in the section ‘solid solution spectroscopy’) are not present. Particular bands due to isotopic species of light atoms as B, C, N, O, etc. may be possible in the case of molecular compounds but the respective modes are not characteristic of special bonding phenomena. Isotope dilution techniques should not be confused with isotope exchange experiments. In the case of such experiments, the atoms of an element are substituted by a heavier or lighter isotope as completely as possible. In this case, the frequencies of bands, in the vibrational modes of which motions of the respective atoms are involved, are shifted (kinetic energy, G matrix or ‘effective inverse mass matrix’ of the respective normal mode [8]). Hence, correct assignment of these bands can be achieved. Such studies have been performed with isotopes of relatively light atoms like Li, Be, B, C, N, O, F, etc. and, hence, relatively large isotopic frequency shifts, but recently also with heavier atoms as metal atoms like Mn, Fe, Zn, Pd, etc. [9,10]. It should, however, be not forgotten that the respective atoms may strongly influence also modes, which do not show isotope exchange frequency shifts because the respective atoms are not moved (potential energy, F matrix of the respective normal modes [8]).

3. Solid solution spectroscopy Ions incorporated on a lattice site of a crystalline matrix are distorted and compressed by the lattice potential at the respective site as compared to the free entities in solution or

in the gas or liquid phase. This results in pressure hardening (or softening) of the intramolecular bonds as well as in change of the free-ion symmetry to the site symmetry of the host lattice. Infrared and Raman spectra of matrix isolated guest ions display these effects explicitly. In addition, the small half-width of bands due to matrix isolated entities enables both the frequencies and the anharmonicity constants of the respective modes, etc. to be established very precisely [11,12]. The pressure of the lattice potential at the respective lattice site is revealed by a mostly high-frequency shift of the internal vibrations of the incorporated entities. The distortion of the incorporated ions as compared to the respective free entities is shown by both the amount of splitting of degenerated modes and the intensity of modes forbidden in the free-ion case. Distortion of matrix isolated ions or molecules revealed from infrared and Raman spectra is called energetic distortion as distinguished from geometric distortion, that is change of distances and angles as established by structural studies [13]. We performed such solid solution spectroscopic experiments on monoclinic Ba(ClO3)2·H2O-type and orthorhombic Sr(ClO3)2-type host crystals with incorporated ClO2 3, 2 BrO2 3 , and IO3 guest ions [14 – 16] as well as on monoclinic MgSO4·H2O(kieserite)-type host crystals with SO22 and 4 2 SeO22 4 guest ions [17]. The lattice sites of the ClO3 ions of Sr(ClO3)2 and of Ba(ClO3)2·H2O are strongly distorted with C1 site symmetry as compared to C3v symmetry of free halate ions, the SO22 ions of kieserite-type compounds 4 possess C2 site symmetry instead of Td symmetry of free sulfate and selenate ions (Table 1). Sr(ClO3)2-type, Ba(ClO3)2·H2O-type, and kieserite-type compounds with matrix isolated halate and sulfate (and metal) guest ions (and HDO molecules), respectively, were obtained by crystallization from aqueous solutions containing small amounts (3 –10%) of the guest ions (and, if necessary, by dehydration of the primarily precipitated hydrates in a vacuum) [16]. The infrared and Raman spectra at ambient and liquid nitrogen temperature were recorded on a Perkin– Elmer model 580 grating spectrometer, a Bruker model IFS 113v Fourier transform interferometer, a Coderg model T800 laser Raman spectrometer, and a Dilor model OMARS 89 multichannel Raman spectrograph resolution , 2 cm21). For excitation, the 514.5 nm line of an Arþ ion laser was used. The modes of the internal vibrations of C3v symmetry halate ions and Td symmetry sulfate ions are shown in

Table 1 22 Correlation of the internal modes of pyramidal XO2 3 and tetrahedral XO4 ions in the point groups C3v and Td and in the site groups C1 and C2 ; respectively XO2 3

C3v

C1

XO22 4

Td

C2

n1 ; nXOðsymÞ n2 ; dXO3 ðsymÞ n3 ; nXOðasymÞ n4 ; dXO3 ðasymÞ

A1, Raman A1, Raman E, IR, Raman E, IR, Raman

A, IR, Raman A, IR, Raman A, IR, Raman A, IR, Raman

n1 ; nXOðsymÞ n2 ; dXO4 ðsymÞ n3 ; nXOðasymÞ n4 ; dXO4 ðasymÞ

A1, Raman E, Raman F2, IR, Raman F2, IR, Raman

A, IR, Raman 2A, IR, Raman A þ 2B, IR, Raman A þ 2B, IR, Raman

H.D. Lutz / Journal of Molecular Structure 704 (2004) 71–78

Table 1. In the case of C1 and C2 site symmetries, all vibrations of halate and sulfate (selenate) ions are allowed in infrared and Raman experiments in contrast to free XO2 3 and XO22 4 ions. In the latter case, some modes are forbidden in infrared and Raman spectra (Table 1) and the modes of E and F symmetry are degenerated. In addition, in Raman experiments, the symmetric stretching modes n1 of A1 symmetry are the strongest bands of all. As revealed from the literature, in the spectra of solid halates and sulfates, this is very frequently not the case. The reason is discussed below. Raman spectra of chlorates and bromates with matrix isolated halate guest ions in the XO stretching mode regions of the guest ions are shown in Figs. 1 and 2. The respective infrared spectra are similar. On comparing the spectra one can see three things. First, the splitting of three XO stretching modes is larger in the case of the strontium chlorate host that is higher energetic distortion, than in both the respective barium compound and in the monohydrate host. Second, the mean values of the wave numbers of the XO stretching modes (Table 2) are larger in the strontium chlorate host than in the barium chlorate and in the monohydrate host. Third, the intensities of the three Raman bands are similar in the case of the anhydrous chlorate hosts but different in the case of barium chlorate monohydrate host. The reason of these findings are (i) the higher lattice potential at the ClO2 3 sites of anhydrous strontium chlorate as compared to the barium compound and to the monohydrate and (ii) the reduced intramolecular coupling of the three XO stretches. The higher lattice potential of the strontium compound, which is caused by the smaller unit

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2 Fig. 2. Raman spectra (90 K) of matrix isolated BrO2 3 and IO3 guest ions (1–2 mol%) in Ba(ClO3)2·H2O host crystals [14–16].

cell, and, hence, the bigger electrostatic field, and the higher compression of the halate ions at the respective lattice site give rise to stronger site group splitting and higher pressure induced high-frequency shifts of the respective modes as compared to the other compounds under discussion. As one can see in Fig. 1, there are three BrO and IO stretching and bending modes. Which are the symmetric stretching mode n1 and the two components of the asymmetric stretching mode n3 ? In order to decide which of the three stretching bands has to be assigned to the n1 mode, we performed single-crystal Raman experiments on strontium chlorate with matrix isolated bromate and iodate ions [18]. The result was that in the case of bromate and iodate ions in a strontium chlorate host lattice, the distortion of these ions is so large that the intramolecular coupling of the three BrO and IO stretching modes to a symmetric n1 and an asymmetric n3 is strongly reduced and, hence, each of the three bands can mainly be assigned to one of the three BrO and IO arms of the respective halate ions as shown Table 2 Assignment of the XO stretching modes (cm21) of matrix isolated BrO2 3 2 and IO2 3 guest ions and of the ClO3 host ions in orthorhombic Sr(ClO3)2 to the three ClO groupings in the host crystal structure as established by single crystal Raman measurements and mean values of the XO stretching modes nXO (Fig. 1) [18]

Fig. 1. Raman spectra (90 K) of matrix isolated (1–2 mol %) in Sr(ClO3)2 host crystals [15,16].

BrO2 3

and

IO2 3

guest ions

BrO2 3 IO2 3 ClO2 3

ClO(1)

ClO(2)

ClO(3)

nXO

781 785 893

825 824 945

874 862 1021

827 824 953

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H.D. Lutz / Journal of Molecular Structure 704 (2004) 71–78

Fig. 3. IR spectra (KBr discs) in the SO stretching mode region of neat kieserite-type MnSO4·H2O and of matrix isolated SO22 guest ions in 4 kieserite-type MnSeO4·H2O host crystals (Co2þ and Ni2þ: metal guest ions; dotted line: ambient temperature, full line: liquid nitrogen temperature) [17].

in Table 2. The less reduced intramolecular coupling of the three XO stretches in the case of the halate hydrates is evidenced by the much bigger intensity of one, i.e. the n1 like (?), of the three Raman bands (Fig. 2). Another question in this context was the relative order of the XO stretching modes of halate ions. It has been known for a long time that for chlorate ions the wave number of the symmetric stretching mode n1 is smaller than that of the asymmetric mode n3 : The opposite was observed for iodate ions with n3 smaller than n1 (Fig. 2). What is the matter with bromate ions? There is some controversy in the literature with n1 . n3 ; n1 ¼ n3 ; and n1 , n3 : The main criterion of assignment was the relative intensity of the respective Raman bands in various solid bromates and bromate aqueous solutions. But the matter is that in solid bromates

intramolecular coupling of the BrO stretching modes is reduced as discussed above. In aqueous bromate solutions, the BrO2 3 ions obviously possess C3v symmetry. Raman spectroscopic studies of such solutions resulted in accidental degeneration of n1 and n3 of BrO2 3 ions with very low intensity of the n3 band [19,20]. Solid bromates with undistorted BrO2 3 ions are scarce. The only bromates with the BrO2 3 ions on C3 sites are, to our knowledge, the hexagonal bromate hexahydrates of magnesium, cobalt, nickel, and zinc [21,22].The Raman spectra of these compounds display a very strong peak in the BrO stretching mode region due to n1 but no band, which can unequivocally assigned to n3 : The respective infrared spectra are relatively complicated despite the high site symmetry of the bromate ions because of superimposition of the BrO stretching modes with the librational modes of the hydrate H2O molecules. As shown by model calculations [22], the relative order of the BrO stretching modes in the bromate hexahydrates is n1 , n3 as for chlorate ions in contrast to bromate ions in solution with n1 equal to n3 : Infrared spectra in the SO stretching modes region of kieserite-type selenates with sulfate (and metal) guest ion as well as of a neat kieserite-type sulfate are shown in Fig. 3 [17]. The bands in the range of 1008– 1015 cm21 (90 K) are due to the symmetric stretching or breathing mode n1 of the tetrahedral sulfate ions and the broad band with up to three components at 1084– 1147 cm21 (neat MnSO4·H2O) and the three sharp bands in the range of 1090 – 1176 cm21 (matrix isolated SO22 ions) have to be ascribed to the 4 respective asymmetric stretching modes n3 : The relatively large intensities of the symmetric stretching modes n1 ; which are not allowed with IR experiments for free sulfate ions with Td symmetry, especially in the case of the neat sulfates, and the great site group splitting of the asymmetric tretches n3 display the relatively large energetic distortion of the sulfate ions in kieserite-type compounds, which is larger than expected from the structure data (s. Table 3) [23,24]. The broadening of the n3 stretching modes in the spectra of the neat sulfates is caused by coupling, especially TO/LO splitting effects [5,17]. In addition to the sharpening of the three n3 stretching modes, the main differences of the SO stretching modes of

Table 3 Structural data of kieserite-type MXO4 ·H2 O compounds [6,7], rX – O and rM – O : mean bond lengths (pm) of the XO22 4 tetrahedra and the MO6 octahedra, aX – O : mean angles (deg) of the XO22 tetrahedra, V=z (pm3): unit-cell volumes per XO22 ion and H2O molecule, respectively, Drmax and Damax : maximum 4 4 differences of the respective bond lengths and bond angles [23,24] Compounds

rX – O

aX – O

rM – O

V=106 z

Drmax XO22 4

Damax XO22 4

Drmax MO6

MnSeO4·H2O CoSeO4·H2O NiSeO4·H2O ZnSeO4·H2O MnSO4·H2O CoSO4·H2O NiSO4·H2O ZnSO4·H2O

163.7 163.3 163.8 163.6 147.1 147.4 147.2 147.5

109.59 109.71 109.75 109.65 109.46 109.51 109.56 109.43

218.4 210.1 206.5 210.3 218.1 209.4 206.1 209.4

103.01 95.96 92.82 96.38 95.28 88.34 85.47 88.44

1.7 1.7 1.8 1.3 1.5 1.7 1.8 1.4

2.04 1.93 2.90 2.28 1.66 1.24 1.94 1.41

18.2 14.4 11.1 16.4 16.1 12.0 8.0 12.9

H.D. Lutz / Journal of Molecular Structure 704 (2004) 71–78

SO22 guest ions in kieserite-type selenate hydrates 4 and SO22 ions in the neat sulfates are the much lower 4 intensities and the lower wave numbers of the symmetric stretches n1 : The former result reveal that in the case of selenate hydrate matrices distortion and deviation of the sulfate guest ions from Td symmetry is much smaller than in the neat sulfates. The low-energy shift of the n1 by about eight wave numbers indicates smaller lattice repulsion at the C2 lattice site of the selenate host than in the neat sulfates. Both results are reasonable because the space in the voids at the respective lattice sites of the selenate host compounds

Table 4 Symmetric and asymmetric SO stretching modes n1 and n3 (Table 1; 90 K cm21), mean values n3 of the asymmetric SO stretching modes, separations n3 2 n1 ; Dnac (site group splitting), and Dnmax (differences between the symmetric SO stretching mode n1 and the highest energy component of the asymmetric SO stretching mode n3 ) of matrix-isolated SO22 4 guest ions in kieserite type selenate monohydrate matrices (partly in the presence of matrix-isolated M 02þ guest ions; in parentheses: differences of the SO stretching modes (cm21) of the SO22 4 guest ions and of those of the neat sulfates, respectively) [17] Guest ions Host lattices Mn2þ

Co2þ

Ni2þ

Zn2þ

n3

MnSeO4·H2O 1165 1125 1091 CoSeO4·H2O 1169 1129 1091 ZnSeO4·H2O 1169 1124 1090

n1

n3

n3 2 n1 Dnac Dnmax

1010 (25)

1127 117 (9) (14)

74 (11)

155 (23)

1008 (27)

1130 122 (12) (19)

78 (15)

161 (29)

1010 (25)

1128 118 (10) (15)

79 (16)

159 (27)

MnSeO4·H2O 1170 1008 1129 (211) 1091 CoSeO4·H2O 1173 1005 1131 (214) 1181 ZnSeO4·H2O 1169 1007 1123 (212) 1088

1130 122 (21) (10)

79 (17)

162 (19)

1128 123 (23) (11)

92 (30)

168 (25)

1127 (24)

120 (8)

81 (19)

162 (19)

MnSeO4·H2O 1167 1008 1128 1127 (212) (28) 1090 CoSeO4·H2O 1172 1005 1127 1125 (215) (29) 1083 ZnSeO4·H2O 1168 1008 1126 1123 (212) (210) 1087

120 (4)

77 (7)

159 (14)

122 (6)

89 (19)

167 (22)

1128 120 (2) (11)

76 (3)

159 (13)

1128 124 (2) (15)

83 (10)

166 (20)

73 (0)

152 (6)

MnSeO4·H2O 1167 1008 1126 (29) 1091 CoSeO4·H2O 1170 1004 1126 (213) 1087 ZnSeO4·H2O 1162 1010 1121 (27) 1089

1124 (22)

114 (5)

75

should be larger than that of the respective sulfate parent compounds. As shown in Table 4, the site group splitting of n3 Dnac and the Dnmax values, that are the differences between the n1 mode and the highest energy n3 mode, are obviously largest, if both metal ions, that is the guest and the host metal ion, are transition metal ions like cobalt with ligand or crystal field stabilization energies CFSE unlike zero. In this case, the MO6 and M 0 O6 octahedra of the kieserite type compounds are additionally stabilized compared to metal ions without ligand field stabilization like zinc and in part manganese. This means CoO6 octahedra resist to angular deformations in a higher degree and, hence, cause stronger distortion of the sulfate ions as ZnO6 octahedra, which allow smaller distortion of ions. the SO22 4 Possible distortion of tetrahedral entities to symmetries , Td may be due to both different bond lengths and different bond angles of these units, respectively. Hence, intramolecular coupling of the four stretching modes is more or less reduced with respect to the respective site group. Thus, in a similar manner as established for halate ions two or more stretching modes can be mainly assigned to a special XO arms. Without correct assignment of the observed bands via Raman single crystal experiments or high-level normal coordinate analyses, however, it is very difficult to decide the vibrational situation in detail. Thus, site group splitting

Fig. 4. IR spectra (90 K, Fluorolub mull) in the OD stretching mode region of Ba(ClO3)2·H2O-type halate monohydrates containing matrix isolated X 0 O2 3 halate guest ions and HDO guest molecules (isotopically dilute samples) (*: OD stretching modes of D2O molecules, affected by halate guest ions, **: OD stretching modes of HDO molecules not) [15,26].

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H.D. Lutz / Journal of Molecular Structure 704 (2004) 71–78

Fig. 5. Raman spectra (90 K) in the OD stretching mode region of Ba(ClO3)2·H2O-type halate monohydrates containing matrix isolated HDO 0 molecules and X 0 O2 3 halate and M metal guest ions, respectively (for further explanation see Fig. 4) [15,27].

Dnac ; maximum splitting Dnmax ; splitting between the components n3a; n3b; and n3c [11,12,25] (if n3 and n1 can still be differentiated), Fermi resonance phenomena, etc. are in my opinion not very useful in this connection.

4. Double matrix spectroscopy So-called double matrix spectroscopic experiments were first performed in the eighties on the isostrucutral series M(XO3)2·H2O with M ¼ Ba, Sr, and Pb and X ¼ Cl, Br, and I [15,26,27]. We incorporated small amounts of both bromate ions and HDO molecules simultaneously into

barium chlorate monohydrate and inversely chlorate ions and HDO molecules into the isostructural bromate monohydrate and in addition all other possible combinations. Infrared and Raman spectra of some examples are shown in Figs. 4 and 5. The large peaks are due to the OD stretching modes of the about 10% HDO molecules in the halate monohydrate. There is only one peak because the hydrate H2O molecules in this compound are symmetric with equally strong hydrogen bonds of the two hydrogen atoms [15,26]. The two small peaks labelled with stars are due to the symmetric and the asymmetric OD stretching modes of D2O molecules also present in very small amounts. The two other small OD stretching modes observed are caused by HDO molecules with one hydrogen or deuterium atom bonded to a bromate or a chlorate guest ion present in about 3%. Such experiments are, in particular, appropriate for determining the hydrogen bond acceptor capability of hydrogen bond acceptor groups [4,7,28]. Thus, bromate (and iodate) ions are stronger hydrogen bond acceptors than chlorate ions. Hence, the peak at 2576 cm21 in the Ba(ClO3)2·H2O sample with BrO2 3 guest ions is due OD· · ·OBrO2 2 hydrogen bonds in this compound, which are stronger than the OD· · ·OClO2 2 hydrogen bonds of the host chlorate. The peak at 2622 cm21 has to be ascribed to the OD stretching modes of OD· · ·OClO2 2 hydrogen bonds of HDO molecules with one hydrogen atom involved in OH· · ·OBrO2 2 hydrogen bonds. These stronger bonds compared to the OD· · ·OClO2 2 hydrogen bonds weaken the bonds of the other hydrogen or deuterium atom as described by the so-called donor competitive effect [28 – 31]. In a similar way, the two bands at 2608 and 2560 cm21 of the bromate hydrate host can be explained. The peak at 2608 cm21 is due to the weaker OD· · ·OClO2 2 hydrogen bonds of the chlorate guest ions and the peak at 2560 cm21 has to be ascribed to OD· · ·OBrO2 2 hydrogen bonds of HDO molecules with the hydrogen atom involved in a weaker OD· · ·OClO2 2 hydrogen bonds to chlorate guest ions. Recently, we performed similar studies on kieserite-type sulfate and selenate monohydrates [32]. In passing is should be noted that kieserite-type monohydrates belong to

Table 5 OD stretching modes (nOD ; cm21) of isotopically dilute samples (5–7% D) of kieserite-type hydrates MXO4 ·H2 O containing matrix isolated X 0 O22 4 guest ions (X 0 ¼ S0 and Se0 ), differences Dn between the wave numbers nOD due to the X 0 O22 4 guest ions and those of the parent compounds (see text) and those of the host compounds (in parentheses), respectively, unit-cell volumes V; and mean distances [23,24] between the metal ions and the oxygen atoms of the hydrate H2O molecules rM – Ow [32] Compound

MnSeO4·H2O CoSeO4·H2O ZnSeO4·H2O MnSO4·H2O CoSO4·H2O ZnSO4·H2O NiSO4·H2O

Host lattices 295 K

90 K

S0 O22 4 90 K

2364 2361 2328 2390 2390 2349 2339

2346 2347 2316 2368 2374 2336 2319

2390 2390 2368 – – – –

Se0 O22 4 90 K

2325 2333 2302 –

Dn

V 106 pm3

rM – Ow pm

222 (34) 216 (43) 232 (52) 21 (243) 14 (241) 14 (234) –

412.0 383.8 385.5 381.1 353.4 353.8 341.9

229.5 219.4 220.6 228.4 217.5 217.7 211.5

H.D. Lutz / Journal of Molecular Structure 704 (2004) 71–78

Fig. 6. IR spectra (90 K, Fluorolub mull) in the OD stretching mode region of Ba(ClO3)2·H2O-type halate monohydrates containing matrix isolated M 0 metal guest ions and HDO guest molecules (For further explanation see Fig. 4) [27].

Fig. 7. Part of the crystal structure of kieserite-type monohydrates [23,24] 0 2þ containing both X 0 O22 guest ions (dashed lines: hydrogen bonds). 4 and M

hydrates with H2O bending modes below 1500 cm21 instead of 1600– 1700 wave numbers as in most hydrates [33,34]. In Table 5, infrared spectroscopic data (90 K, selfdeconvolution) of kieserite-type compounds with sulfate or selenate guest ions are compiled; for illustrations of the spectra see Ref. [32]. The small additional bands below or above the OD stretching mode of the host compound are

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due to HDO molecules forming hydrogen bond to sulfate or selenate guest ions. This additional mode is observed at lower wave numbers in the case of selenate guest ions hydrogen bonds are stronger than because OD· · ·OSeO22 3 OD· · ·OSO22 3 hydrogen bonds and at higher wave numbers in the case of sulfate guest ions. The hydrogen bond capability of sulfate ions is smaller than that of selenate ions. These results were expected on the background of our knowledge on hydrogen bond strength discussed before. The relative acceptor capabilities of various ions and molecules thus established and with other methods are reported in Refs. [4,7,28,35,36]. Double matrix spectroscopic studies are not only possible with anionic guest ions. In a similar way, small amounts of both metal guest ions and HDO molecules can be incorporated in a suitable matrix. We performed such studies within the isostructural halate monohydrate [15,27] and kieserite-type series [32] discussed above. In the case of incorporation of HDO molecules and small amounts of lead ions in barium chlorate monohydrate, the HDO molecules attached to lead guest ions form stronger hydrogen bonds than those bonded to the barium host ions (Figs. 5 and 6). This is shown by the red shifted OD stretching mode at 2553 cm21 as compared to 2616 cm21 (IR, see Fig. 6) of the respective host compound band. In the case of incorporation of barium ions in the isostrucutral lead bromate monohydrate, the HDO molecules attached to barium guest ions form much weaker hydrogen bonds (as shown by the small band at 2608 cm21) than the HDO molecules attached to the lead host ions (big band at 2529 cm21). This means the synergetic effect [4,7,28,36] of lead ions is much stronger than that of barium ions. The data of analogous infrared spectra of kieserite-type compounds with metal guest ions, namely the wave numbers of the observed OD stretching modes and their differences to those of the respective host and parent compounds, are compiled in Table 5. For illustration of the spectra, see Ref. [32]. Parent compounds mean, for example, manganese sulfate monohydrate compared to manganese guest ions in other sulfate monohydrates. Interpretation of the wave numbers of the small additional bands due HDO molecules attached to one guest and one host metal ion (Fig. 7) is somewhat more complicated than in the case of halate monohydrates with monodentately bonded H2O molecules.

Table 6 OD stretching modes (cm21, 90 K) of isotopically dilute samples (5% D) of kieserite-type hydrates MSeO4·H2O containing matrix isolated M 0 guest ions (in parentheses: differences Dn between the wave numbers nOD due to the metal guest ions and those of the parent compounds (see text) and those of the host compounds, first and second columns, respectively) [32] Host

Host

Mn

Co

Zn

Ni

MnSeO4·H2O CoSeO4·H2O ZnSeO4·H2O NiSeO4·H2O

2368 2374 2336 2319

– – 2354 (214, 2 18) 2347 (221, 2 28)

– – 2359 (215, 2 23) 2356 (218, 2 37)

2340 (4,28) 2349 (13,25) – –

2327 (8,41) 2336 (17,38) – –

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H.D. Lutz / Journal of Molecular Structure 704 (2004) 71–78

First, let us discuss the frequency shifts compared to the respective host compounds (second columns in the parentheses of Table 6). Here, the size of the respective metal ions and, hence, the respective M – O distances are important (Table 5). Larger sizes of the guest metal ions as compared to the host metal ions and, hence, larger M – O distances mean weaker M – O bonds, weaker synergetic effect, weaker hydrogen bonds, and subsequently higher OD stretching modes of the HDO molecules attached to the guest metal ions and vice versa. Now, let us discuss the wave number differences to the respective parent compounds. There are two criteria that must be considered. First, the different bonding interactions, namely M 0 MOwater· · ·O and M2water· · ·O interactions (Fig. 7), and second, the different repulsion potentials of the lattice at the respective H2O molecule sites, which inversely correlates with the unit cell volumes of the kieserite-type compounds. The smaller the unit-cell volume, the bigger the repulsion potential. On the basis of the former criterion, the wave numbers of the bands due to the M’MOwater· · ·O bonds are expected to be the mean values of the stretching modes due to M2Owater· · ·O and M 0 2Owater· · ·O hydrogen bonds, both considered as neat compounds. The latter criterion should give rise to smaller wave numbers of the guest ion induced OD stretching modes as compared to the mentioned mean values if the unit cell volumes of the respective parent compounds are smaller than those of the host compounds, that is there are decreased repulsion potentials, and vice versa. These predictions, especially the latter one, agree with the observed Dn values shown in the first columns in the parentheses of Table 6. They are always smaller than the wave number differences of the OD stretching modes of the two parent compounds. This means the Dn values are larger than the mean values mentioned above if the unit cell volumes of the parent compounds are larger than those of the host compounds. Further double matrix spectroscopic experiments in the field of hydrogen-bond research with other compounds have been established by Moon et al. [37], Afanasiev et al. [38], Ramı´rez et al. [39], and Stoilova et al. [40,41].

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]

[28] [29] [30] [31] [32] [33] [34]

5. Conclusion [35]

Crystal matrix spectroscopic experiments give a lot of new insights. I have shown some examples of matrix and double matrix spectroscopic experiments performed in our laboratory. These techniques reveal very many details on both symmetry and the geometric and energetic situation at the various lattice sites of crystalline solids in contrast to neat compound spectroscopy. In the latter case, most of this information is hidden by solid state collective effects.

[36] [37] [38] [39] [40] [41]

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