Infrared matrix isolation studies of the acetohydroxamic acid complexes with HF and HCl

Journal of Molecular Structure 692 (2004) 163–168 www.elsevier.com/locate/molstruc

Infrared matrix isolation studies of the acetohydroxamic acid complexes with HF and HCl Magdalena Sałdyka, Zofia Mielke* Department of Chemistry, University of Wrocław, Joliot-Curie 14, 50-383 Wrocław, Poland Received 30 December 2003; revised 30 December 2003; accepted 21 January 2004

Abstract Argon matrix infrared spectra of the complexes formed between acetohydroxamic acid (CH3CONHOH) and hydrogen halides (HF, HCl) have been recorded. The experimental results indicate formation of a strong binary complex in which the hydrogen halide molecule acts as a proton donor towards the carbonyl group of acetohydroxamic acid. The H – X stretches and several perturbed CH3CONHOH vibrations were identified for the two HX complexes; for the HF complex the two librational HF modes were also observed. q 2004 Elsevier B.V. All rights reserved. Keywords: Acetohydroxamic acid; Hydrogen halides; Hydrogen bond; Infrared spectra; Matrix isolation

1. Introduction Hydroxamic acids (RCONHOH) have recently focused a lot of attention because of their implication in a wide spectrum of biological activities [1,2]. Among the problems that attracted most attention of the researchers is the identity of the preferred structure of the hydroxamic acids and their acidity. The theoretical and experimental works performed so far indicates that both the stability of different isomers of hydroxamic acids [3 –5] and their behaviour as N- or O-acids [6 – 9] are strongly dependent on the environment. Our recent FTIR studies of formo- and acetohydroxamic acids [10,11] showed that the two acids trapped from the gas phase into solid argon or nitrogen exist in matrices preferentially in the keto form with an intramolecular hydrogen bond. Hydroxamic acids are N-hydroxy substituted derivatives of amides and involve the fragment of the simplest protein structure HNCyO. The hydrogen bonding abilities and the site of protonation for the amides [12] have been examined extensively in the literature due to their biological importance. It is now a well-recognized fact that in the amide complexes with strong proton donors apart from the carbonyl oxygen also the nitrogen atom may act as proton acceptor site [12 – 14]. In addition to the two basic centres * Corresponding author. Tel.: þ 48-71-3757475; fax: þ 48-71-3282348. E-mail address: [email protected] (Z. Mielke). 0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2004.01.026

characteristic for the HNCyO group, the hydroxamic acids have the third basic centre, which is the oxygen atom of hydroxyl group. Our recent infrared matrix isolation and theoretical studies of the formohydroxamic acid complexes with hydrogen halides demonstrated that the carbonyl oxygen remains the predominant basic centre in the simplest representative of hydroxamic acids [15]. In this paper we present the results of the infrared matrix isolation studies of the acetohydroxamic acid complexes with hydrogen fluoride and hydrogen chloride. The spectra and structure of the energetically favoured complexes trapped in the matrices are determined and discussed with respect to basic properties of acetohydroxamic (AHA) and formohydroxamic (FHA) acids.

2. Experimental Acetohydroxamic acid was commercially available (99% Fluka). The HF/Ar or HCl/Ar and CH3CONHOH/Ar gas mixtures were codeposited simultaneously through two separate spray-on lines. Acetohydroxamic acid was evaporated from the bulb connected to the cryostat via tube. Both bulb and tube were made from stainless steel and were heated to 70 8C during matrix deposition. The monomer concentration was controlled by comparing the spectra obtained at different deposition conditions. The HF/Ar

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and HCl/Ar mixtures were prepared by standard manometric technique; the concentration of HF/Ar and HCl/Ar varied in the range 1/100 –1/2000. Gold-plated copper mirror was used as a sample holder and was maintained at 20 K (12 K for IR measurements) by means of a closed cycle helium refrigerator (Air Products, Displex 202A). Infrared spectra were recorded in a reflection mode with a resolution 0.5 cm21 by means of a Bruker 113v FTIR spectrometer using liquid N2 cooled MCT detector (4000 – 600 cm21).

3. Results The spectra of AHA/Ar and HX/Ar (XyF, Cl) matrices agree well with those previously reported [11,16,17]. The spectra of AHA/HX/Ar (XyF, Cl) matrices showed a number of prominent new absorptions as compared to the spectra of HX/Ar and AHA/Ar matrices. Most of the product absorptions appeared in the vicinity of the bands due to the acetohydroxamic acid monomer. As was shown earlier [11], the AHA monomer exists mainly as the 1Z keto tautomer in low temperature matrices. The product bands occurring in the vicinity of the 1Z AHA isomer were assigned to the perturbed vibrations of this molecule in the complex. Additional product bands were also observed for the bonded HF and HCl molecules as discussed below. The representative regions of the CH3CONHOH/HX/Ar spectra are shown in Figs. 1 –6.

Fig. 1. The nNH and nOH region in the infrared spectra of matrices: (a) CH3CONHOH/Ar, (b) CH3CONHOH/HCl/Ar, where HCl=Ar ¼ 1=300; (c) CH3CONHOH/HF/Ar, where HF=Ar ¼ 1=400:

Fig. 2. The Amide I ðnCyOÞ region in the spectra of the same matrices as presented in Fig. 1.

3.1. Acetohydroxamic acid – hydrogen fluoride complexes In the spectra of the AHA/HF/Ar matrices the new absorptions appeared at 3503.0, 3501.0, 1656.0, 1641.0, 1456.0, 1415.0, 1399.5, 1088.5, 1084.0, 1002.0, 999.5 and 913.0 cm21, close to the CH3CONHOH monomer absorptions (Figs. 1– 5). The band at 1456.0 cm21 showed

Fig. 3. The dCH3 and dNOH regions in the spectra of the same matrices as presented in Fig. 1.

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Fig. 4. The rCH3 region in the spectra of the same matrices as presented in Fig. 1.

a shoulder at 1459.5 cm21. In addition, strong, broad band occurred at 3243.0 cm21 with a shoulder at 3227.0 cm21 (Fig. 1) and two strong, characteristic bands were observed at 811.0 and 765.0 cm21 (Fig. 5). The relative intensities of the new product bands were not sensitive to matrix annealing and were independent of matrix concentration.

165

Fig. 6. The nHCl region in the spectra of matrices: (a) HCl=Ar ¼ 1=300; (b) CH3CONHOH/HCl/Ar, where HCl=Ar ¼ 1=300:

3.2. Acetohydroxamic acid – hydrogen chloride complexes In the spectra of AHA/HCl/Ar matrices the product absorptions were observed at 3490.5, 1635.0, 1455.0, 1407.5, 1399.0, 1087.0, 1083.0 and 998.5 cm21 in the vicinity of the CH3 CONHOH monomer vibrations (Figs. 1– 6). The band at 1407.5 cm21 showed a shoulder at 1413.0 cm21. An additional broad and diffuse band was detected at ca. 2400 cm21 (Fig. 6). The above absorptions were observed for all concentrations studied. With the increase of HCl concentration in the matrix, all the product bands grew at the same rate. The frequencies of all product absorptions observed in the spectra of AHA/HF/Ar and AHA/HCl/Ar matrices are shown in Table 1.

4. Discussion 4.1. Spectral characteristics

Fig. 5. The nNO region in the spectra of the same matrices as presented in Fig. 1.

4.1.1. Acetohydroxamic acid –hydrogen fluoride complexes The set of bands observed in the spectra of the AHA/HF/Ar matrices at 3503.0, 3501.0, 1656.0, 1641.0, 1456.0 (with a shoulder at 1459.5 cm21), 1415.0, 1399.5, 1088.5, 1084.0, 1002.0, 999.5 and 913.0 cm21 close to the acetohydroxamic acid absorptions and the characteristic product bands observed at 3243.0 (with a shoulder at 3227.0 cm21), 811.0 and 765.0 cm21 can be assigned with confidence to the 1:1 CH3CONHOH· · ·HF complex. The relative intensities of the bands do not depend on matrix concentration and remain constant after matrix annealing.

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Table 1 Observed frequencies and frequency shifts, Dn ¼ ncomplex 2 nmonomer ; for acetohydroxamic acid–hydrogen fluoride and acetohydroxamic acid–hydrogen chloride complexes in solid argon (cm21) Monomer CH3CONHOH

CH3CONHOH· · ·HF

n

n

Dn

n

Dn

3454.0 3323.0 3009.0 3002.0 2948.0 1690.5 1513.5, 1505.5 1454.0 1428.5 1406.0 1391.0 1341.5 1081.5 1069.0 992.5 904.5 651.5 643.5 HF/HCl 3919.5/2887.0

3503.0, 3501.0

þ 49.0

3490.5

þ 36.5

1656.0, 1641.0

234.5

1635.0

255.5

þ5.5

1455.0

þ 1.0

1459.5 sh, 1456.0

CH3CONHOH· · ·HCl

Assignment

1399.5 1415.0

26.5 þ 24.0

1399.0 1413.0 sh, 1407.5

27.0 þ 22.0

1088.5 1084.0 1002.0, 999.5 913.0

þ7.0 þ 15.0 þ9.5 þ8.5

1087.0 1083.0 998.5

þ 5.5 þ 14.0 þ 6.0

3243.0, 3227.0 sh 811.0 765.0

2676

The appearance of the 3503.0, 3501.0; 1656.0, 1641.0 and 1002.0, 992.5 cm21 doublets is most probably due to the site splitting. The 3503.0 and 3501.0 cm21 doublet in the spectra of the AHA/HF/Ar matrices is assigned to the perturbed NH stretching vibration in the CH3CONHOH· · ·HF complex. It shows ca. 48 cm21 blue shift with respect to the corresponding band of the AHA monomer. The product bands at 1656.0 and 1641.0 cm21 are due to the perturbed Amide I mode in the bonded AHA molecule. The bands are ca. 42 cm21 shifted towards lower frequencies with respect to the Amide I mode of the AHA monomer. The very strong band observed at 1415.0 cm21 is attributed to the perturbed NOH in plane bending vibration of AHA; this absorption shows 24 cm21 blue shift with respect to the corresponding band of the free AHA molecule. The new bands occurring at 1456.0 cm21 (with a shoulder at 1459.5 cm21) and at 1399.5 cm21, in the region of the CH3 bending modes of AHA, are assigned to the perturbed das(CH3) and ds(CH3) modes of the CH3CONHOH molecule. The bands are 4 cm21 blue shifted and 6.5 cm21 red shifted, respectively, from the corresponding monomer absorptions. The product bands at 1088.5 and 1084.0 cm21 are attributed to the rocking modes of the CH3 group of AHA, they are 7 and 15 cm21 blue shifted with respect to the corresponding monomer absorptions. A doublet at 1002.0, 999.5 cm21, that appears in the region of the absorptions due to the acetohydroxamic acid associates [11], is assigned to the perturbed NO stretching

,2400

2487

NH stretch OH stretch CH3 asym. stretch CH3 asym. stretch CH3 sym. stretch Amide I (CyO stretch) Amide II (NH bend) CH3 asym. bend CH3 asym. bend CH3 sym. bend NOH bend Amide III (CN stretch) CH3 rock CH3 rock NO stretch C– C stretch Amide IV (OCN bend) CN torsion HX stretch HF libr. sym. HF libr. asym.

mode in the complex. The pair of bands is ca. 8 cm21 blue shifted from the corresponding monomer band. The weak product band at 913.0 cm21, that is 8.5 cm21 blue shifted from the C – C stretching vibration of the AHA monomer, is assigned to the corresponding band in the complex. The strong, broad band observed in the spectra of the CH3CONHOH· · ·HF complex at 3243.0 cm21 with a shoulder at 3227.0 cm21 is assigned with confidence to the perturbed HF stretching vibration. The bands appearing at 811.0 and 765.0 cm21 are attributed to the HF librational modes. The frequencies of the HF modes in the CH3CONHOH· · ·HF complex have close values to the frequencies of the corresponding modes in HF complexes with other carbonyl compounds [13,18,19] (Table 2), and especially with amides. For formamide – HF and acetamide – HF complexes [13] the ns ðHFÞ; nlibr:sym ðHFÞ and nlibr:asym ðHFÞ modes were identified at 3242, 864 and 772 cm21 and at 3160, 848 and 805 cm21, respectively. 4.1.2. Acetohydroxamic acid – hydrogen chloride The set of bands observed at 3490.5, 1635.0, 1455.0, 1407.5 (with a shoulder at 1413.0 cm21), 1399.0, 1087.0, 1083.0 and 998.5 cm21 in the vicinity of the CH3CONHOH bands and the broad band at 2400 cm21 is assigned to the 1:1 CH3CONHOH· · ·HCl complex. The appearance of doublets is attributed to multiple trapping sites in an argon matrix. The absorption due to the perturbed NH stretch occurs in the spectrum of the CH3CONHOH· · ·HCl complex at

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Table 2 The frequencies (cm21) of the ns ðHXÞ and nlib ðHXÞ vibrations in a series of complexes between carbonyl compounds and hydrogen halides in solid argon BASE

H2CO CH3(H)CO CH3COOH CH3COOCH3 (CH3)2CO HCONHOH CH3CONHOH HCONH2 CH3CONH2 CH3CON(CH3)2 HCON(CH3)2 a

PAa (kJ/mol)

713 769 784 822 812 834 854 822 864 908 888

HF complexes

HCl complexes

ns

nlib

Ref.

ns

Ref.

3750 3416 3406 3351 3302 3293.0 3243.0 3242 3160 3072

612; 602 722; 693 736; 699 756; 728 778; 756 796.0; 742.5 811.0; 765.0 864; 772 848; 805 877; 826

[18] [18] [19] [19] [18] [15] This work [13] [13] [13]

2450 2392 ,2380 ,2400

[22] [21] [15] This work

740 ,1600–700

[12] [20]

From Ref. [23].

3490.5 cm21, showing 36.5 cm21 blue shift with respect to the monomer band. The strong band observed at 1635.0 cm21 is assigned to the perturbed Amide I vibration. This vibration is 55.5 cm21 red shifted with respect to the corresponding monomer mode. The NOH in plane bending vibration of the complex is identified at 1407.5 (with the shoulder at 1413.0 cm21) and is ca. 19 cm21 blue shifted from the corresponding absorption of the AHA monomer. The perturbed das(CH3) and ds(CH3) modes of the CH3CONHOH· · ·HCl complex are identified at 1455.0 and 1399.0 cm21, respectively, and are 1 cm21 blue shifted and 7 cm21 red shifted from the corresponding AHA monomer absorptions. The CH3 rocking modes of the bonded CH3CONHOH molecule appear at 1087.0 and 1083.0 cm21 and are 5.5 and 14 cm21 blue shifted from the corresponding absorptions of the free AHA molecule. The 998.5 cm21 band is assigned to the perturbed NO stretching mode in the CH3CONHOH· · ·HCl complex, it exhibits 6 cm21 blue shift with respect to the corresponding monomer mode. The weak and broad band observed in high frequency region at ca. 2400 cm21 is assigned to the perturbed HCl stretch in the complex with acetohydroxamic acid. 4.2. Bonding and structure The acetohydroxamic acid complexes with HF and HCl show similar spectral characteristics. In both HF and HCl complexes the perturbed NH stretching, NOH in plane bending and NO stretching modes of acetohydroxamic acid exhibit blue shifts, whereas the Amide I vibrations show significant red shifts. The ns ðHXÞ vibration is strongly red shifted in both complexes (2 676, 2 487 cm21 in HF, HCl complexes, respectively). The observed perturbations of both submolecules after complex formation indicates the structure in which HX acts as a strong proton donor and CH3CONHOH acts as a proton acceptor. The spectral characteristics of the acetohydroxamic acid complexes with HF and HCl is very similar to

the characteristics of the formohydroxamic acid (FHA) complexes with hydrogen halides [15]. A large red shift of the Amide I mode and relatively small perturbations of the NOH group vibrations confirm similar geometries of the AHA and FHA complexes with HF and HCl. Similarly, like in the FHA – HX complexes, also in the AHA – HCl complexes this is the carbonyl group of AHA that plays a role of proton acceptor for the HX molecule; the structures in which HX is attached to the NOH group of AHA can be safely excluded. The CH3CONHOH molecule is a simple derivative of acetamide which also points out to the oxygen atom of the carbonyl group as the most probable site of attachment of hydrogen fluoride or hydrogen chloride. The red frequency shifts of the perturbed Amide I mode (34.5 and 55.5 cm21 in CH3CONHOH· · ·HF and in CH3CONHOH· · ·HCl complexes, respectively) are characteristic for attachment of HX subunit to the oxygen atom of carbonyl group in amide· · ·HX complexes. For formamide· · ·HF, acetamide· · ·HF and N,N-dimethylacetamide· · ·HF complexes isolated in solid argon the 18, 47 and 32 cm21 red shifts of the Amide I modes were observed after complex formation [13]. In N,N-dimethylacetamide· · ·HCl and N,N-dimethylformamide· · ·HCl complexes isolated in Ar matrices the perturbed Amide I mode was found to be ca. 70 and 40 cm21 red shifted with respect to the corresponding monomer vibration [12,20]. In the argon matrix spectra of formohydroxamic acid· · ·HF and HCl complexes [15] the 20.5 and 31 cm21 red shifts of the Amide I mode were observed, respectively. The 3243.0, 811.0 and 765.0 cm21 bands assigned to the ns ðHFÞ; nlibr:sym ðHFÞ and nlibr:asym ðHFÞ modes in CH3CONHOH· · ·HF complex appear in frequency ranges in which the corresponding HF vibrations of the HF complexes of other carbonyl compounds occur (Table 2). The 676 cm21 red shift observed for the HF stretch in CH3CONHOH· · ·HF complex indicates formation of hydrogen bond of comparable strength to the hydrogen bonds in acetone and formamide complexes with HF. In the latter two complexes the HF stretch is 617 and 677 cm21 shifted

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towards lower frequencies from its monomer value [13,18]. The values of the nlib frequencies identified for AHA complex are in accord with this conclusion. The weak and broad band at 2400 cm21 that is due to the HCl stretch in CH3CONHOH· · ·HCl complex appears at similar frequency as the ns ðHClÞ absorption in the spectra of acetone· · ·HCl system in solid argon (2393 cm21) [21]. In N,N-dimethylacetamide· · ·HCl [13] and N,N-dimethylformamide· · ·HCl [20] a very strong hydrogen bond is formed with the proton shared between the oxygen and chlorine atoms (Table 2). In the argon matrix spectra of the FHA complexes with HF and HCl [15], the ns ðHFÞ; nlibr:sym ðHFÞ and nlibr:asym ðHFÞ modes were identified at 3293.0, 796.0 and 742.5 cm21, respectively. The lower frequency of the HF stretch and higher frequencies of the librational modes in the CH3CONHOH· · ·HF complex than in the HCONHOH· · ·HF one indicate stronger hydrogen bonding in acetohydroxamic acid complex. This fact is also confirmed by stronger perturbations of the intramolecular vibrations of acetohydroxamic acid molecule as compared to the formohydroxamic acid one [15]. The stronger interaction in acetohydroxamic acid· · ·HF complex than in formohydroxamic acid one is in accord with larger proton affinity of the AHA molecule as compared to FHA one (Table 2). The methyl group in CH3CONHOH increases the basic properties of the CO group in this molecule as compared to HCONHOH one. The ns ðHClÞ modes appear in the spectra of the CH3CONHOH· · ·HCl and HCONHOH· · ·HCl complexes [15] at ca. 2400 and 2380 cm21, respectively, as weak and broad absorptions. They do not provide information on the relative strength of interactions in the two complexes and the librational modes were not identified. However, larger perturbations of the CONHOH group vibrations in CH3CONHOH complex than in HCONHOH one indicate that the stronger hydrogen bonding occurs in acetohydroxamic acid· · ·HCl complex.

5. Conclusions FTIR studies have shown that the complexes of acetohydroxamic acid with hydrogen fluoride and hydrogen chloride trapped in argon matrices have a structure in which the hydrogen halide molecule acts as a proton donor toward the carbonyl group of CH3CONHOH that is the main basic centre in the hydroxamic acid molecule. Several perturbed CH3CONHOH vibrations, one HCl (ns ) and three HF

vibrations (ns ; nlibr:sym ; nlibr:asym ) were identified for the hydrogen chloride and hydrogen fluoride complexes. The stronger perturbation of the acetohydroxamic acid vibrations in the hydrogen fluoride complexes indicates higher interaction energy for the hydrogen fluoride complexes than for the hydrogen chloride ones.

Acknowledgements The authors gratefully acknowledge financial support from the Polish State Committee for Scientific Research (Grant KBN No. 3T09 A 062 18).

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