Fourier transform infrared and Raman spectra of N-di-isopropylphosphorylguanidine (DPG)

Spectrochimica Acta Part A 56 (2000) 1563 – 1574 www.elsevier.nl/locate/saa

Fourier transform infrared and Raman spectra of N-di-isopropylphosphorylguanidine (DPG) Claudio A. Te´llez S a,*, Judith Felcman b, Andre´a de Moraes Silva b a

Instituto de Quı´mica, Departamento de Fı´sico-Quı´mica, Uni6ersidad Federal Fluminense (UFF), Morro do Valonguinho s/n, Niteroi-Centro. Cep, 24210 -150 Rio de Janeiro, Brazil b Departamento de Quı´mica, Pontifı´cia Uni6ersidade Cato´lica do Rio de Janeiro, PUC-Rio’ R. Marque´s de S. Vicente 225, Ga´6ea. Cep: 22453 -900 Rio de Janeiro, RJ-Brazil Received 24 September 1999; accepted 2 December 1999

Abstract The Fourier transform infrared and the Fourier transform Raman spectra of N-di-isopropylphosphorylguanidine (DPG) in the solid state and in aqueous solution were recorded and analyzed. Assuming Cs symmetry for different structural fragment of the molecule, the experimental and calculated band assignments of the n(NH), d(HNH), d(CNH), n(CN),n(PN),n(CN), n(PO) and n(OC) normal modes suggested that the DPG exists as a tautomeric contribution of the phosphorylamine (I) and N-phosphorylimine (II) structural forms.

© 2000 Elsevier Science B.V. All rights reserved. Keywords: N-di-isopropylphosphorylguanidine; Infrared and Raman spectra; Tautomerism; Assignment of characteristic frequencies

1. Introduction N-di-isopropylphosphorylguanidine, hereafter abbreviated as DPG belongs to a group of phosphonated (PV) compounds of which nucleic acids are important examples because they play a vital * Corresponding author. Tel.: +55-21-2745689. E-mail address: [email protected] (C.A. Te´llez, S)

role in the biological processes [1]. This phosphorilated compound is a powerful complex forming agent for metal ions [2,3] with ligand action throughout the phosphoryl and NC groups. Thus, it is very similar to phosphocreatine, a natural ligand involved in energy production. From studies involving other phosphorilated compounds such as N- (dialkoxyphosphinyl and -othioyl)-N%-alkyl (aryl) thioureas [4] it can be con-

1386-1425/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 9 9 ) 0 0 2 8 5 - 1

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clude that these compounds when in concentrated solutions and in the condensed phase are associated through intermolecular hydrogen bonds formed with the participation of PNH, CS and PS groups. The structure, acid-base and complexforming properties of N-phosphorylated thiobenzamides were also studied [5]. An experimental and theoretical study of the structure and aggregation of biphenyl guanidine (DPhG) in non-polar and low-polarity solvents was performed by Koll and associated [6]. These authors on the basis of MNDO-PM3 and AM1 methods, had showed that in low-polarity solvents DPhG exists in the form of an asymmetric tautomer, the same as was found in the solid-state structure. In order to understand the tautomerism (RO)2-NHPhU(RO)2HP =NPh, about fifteen compounds containing PN bonds were studied by Kabachnik et al. [7]. They concluded that the structure (RO)2HPNPh exists in all compounds. Studies on tautomerism of substituted trichloroacetamidines were carried out by Moritz [8], which concluded that these compounds exist as a mixture of amino and imino tautomers. Mikolajczyk et al. [9] studied the stereochemistry of the rearrangement of S-phosphorylisothioureas into N-phosphorylthioureas. The tautomerisation of acetylacetone enol was studied by Spyridis e Meany [10] by means of a kinetic method based on the relative positions of equilibrium between the keto and enol tautomers in solvents of varying polarities, and the intramolecular hydrogen bonding of the enol forms of b-ketothioamides-deuterium isotope effects on C-13 chemical shifts was studied by Hansen et al. [11]. Recently, the vibrational spectra of 1,2,4-triazole-thione and its Nmethyl derivatives were studied by A. Elhajii et al. [12]. They found the existence of one of the two predicted tautomeric forms of these compounds. For DPG, X-ray diffraction data had revealed that the bond lengths N (1)-C-N (3) are the same. RMN-P31 studies had not confirmed the existence of proton forming a bond with the N (1). RMN-H1 analysis at low temperature (− 30 and − 60°C) revealed the existence of conformers in the structure (II), due to rotation of the structure around the PN bond [13]. Experimental studies for cianoguanidine in the solid state and in solution were carried out by Sheludyakova et al. [14]. These

authors had confirmed the existence of tautomerism and suggest that the X-ray data showing equivalence in the bond lengths CN and CN is a consequence of the superimposition of the two tautomeric forms. For DPG there is no evidence of tautomerism by vibrational infrared and Raman techniques and up to now, we do not know any infrared or Raman study on the vibrational spectra of DPG with this purpose. The present study is devoted to a vibrational analysis of the more characteristic IR bands and Raman shifts of DPG with the intention to elucidate if the existence of phosphorylamine (II) and N-phosphorylimine (I) tautomers are present in the solid state and in aqueous solution.

2. Experimental N-di-isopropylphosphorylguanidine (DPG) is a phosphorilated guanidine with 223.2 Da of molecular mass and with a melting point between 399– 400° K. The synthesis of DPG was carried out according the procedure proposed by Combie [15] and Souza [13]. Recrystalization was done in absolute ethanol. The characterization of the substance was performed through several techniques: determination of melting point, which was in good agreement with the reference values [10], mass spectrometry, elemental analysis and infrared spectrometry for characterization of the functional groups bands. The mass spectrometry analysis was carried out in a mass spectrometer built in the Van der Graaf Laboratory of PUC-Rio using a dessortion technique of the particles produced by the fragment of fission of the radioactive nucleotide 252 of the Californium element (252Cf-PDMS). The spectrum presented a large peak at 223 corresponding to the molecular mass of the compound. The infrared spectra were recorded on a Perkin– Elmer 2000 FT-IR spectrometer. Data were collected with 0.5 cm − 1 interval with a resolution of 4 cm − 1. The scanning speed was 0.2 cm − 1 and 120 scans were performed. The solid samples were measured in the range of 4000–370 cm − 1 as KBr pellets, and in the region of 710–30 cm − 1 as polyethylene pellets.

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Raman spectra of DPG in solid state and in aqueous solution were also carried out in a NICOLET 950 FT-Raman. The experimental conditions for the measurements in the solid sample were: scan number, 100; collection length, 20 904 s; resolution, 4; laser frequency, 15.7982 cm − 1; Interferogram peak position’ 8192; apodization, Happ –Genzel; detector, InGaAs beam splitter; CaF2 aperture, 150; sample gain, 16; Raman laser current, 8.5; Raman laser frequency, 9393.44 cm − 1. The conditions for aqueous solutions were scan number, 600; collection length, 12541 s; resolution: 4; laser frequency, 15.7982 cm − 1; interferogram peak position, 8192; apodization, Happ – Genzel; detector, InGaAs beam splitter; CaF2 aperture, 150; sample gain, 16; Raman laser current, 8.5; Raman laser frequency, 9303.64 cm − 1.

3. Results

3.1. N –H stretching frequencies Fig. 1 shows the infrared and Raman spectra of solid N-di-isopropylphosphorylguanidine (DPG). The asymmetric and symmetric N – H stretching normal modes of a large number of primary amines absorb in the IR spectra in the region of 3500–3300 cm − 1. Secondary amines present one broad band between 3500 – 3300 cm − 1 and the imine groups present one band at about 3400 – 3000 cm − 1 [16]. In compounds containing the P-NHR (alkyl) group the NH stretching normal modes absorb between 3316 – 3125 cm − 1 [17]. In DPG due to the same bond length between the N1 – C and C–N2 atoms pairs, and based on the assumption of tautomerism in cianoguanidine claimed by Shedudyakova et al. [14], we found arguments in favor of the possibility of tautomerism between the phosphorylamine (I) and the phosphorylimine (II) structures:

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If the structure (II) proved to be the only tautomer present in the solid substance, only the N-H stretching vibrations pertaining to a primary amine, i.e. wavenumbers between 3500–3300 cm − 1 would appear in the spectra. In the infrared and Raman spectra of solid DPG we observed bands at: 3441 (IR), 3315 (IR), 3172 (IR), 3423 (R), 3405 (R), 3317 (R), 3183 (R) cm − 1 and a very weak Raman shift at 3164 cm − 1. Band contour analysis (BCA) of the infrared spectrum in the region of 3550–3000 cm − 1 reveals bands at: 3449 (BCA), 3414 (BCA), 3313 (BCA), 3174 (BCA) and at 3118 (BCA) cm − 1, and the BCA for the Raman spectrum shows bands at: 3412, 3368, 3311 and at 3188 cm − 1. Contrasting with the infrared bands, the Raman shifts have are of very weak intensity. Wavenumbers in the range of 3360–3449 cm − 1 were assigned to the NH stretching of the -NH2 group of the phosphorylimine (II) structure. These wavenumbers follows the Bellamy and Williams equation proposed for primary amines [16]: n(a%)= 345.5+ 0.876n(a¦), see by example the set of wavenumbers 3441 (IR)–3360 (R) cm − 1 and 3405 (IR)–3315 (R) cm − 1, which were assigned to belong to the phosphorylimine (II) structure. N–H stretching bands in aqueous solution are overlapped by the strong Raman shift corresponding to the n(OH) vibrational mode. In aqueous solution of DPG, Raman shifts at 2991 and at 2945 cm − 1 were observed and they correspond to the n(CH) stretching vibrational modes of the methyl groups in the DPG molecule. Our assignments for the n(NH) normal modes were confirmed by normal co-ordinate analysis (NCA) which was carried out for the HNC(NH)(NH2) and NC (NH2)2 fragments of the molecular structure. Fig. 2 shows the IR and Raman spectra together with the BCA bands of solid DPG in the region of 3025–3550 cm − 1. Methyl groups wavenumbers in DPG are listed with the assignment in Table 1.

3.2. CN stretching frequencies Considering the possibility of tautomerism between the phosphorylamine (I) and the phosphorylimine (II) structures, as it is evidenced in the IR and Raman spectra of solid DPG at the region of 3500–3000 cm − 1, it is expected two different

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Fig. 1. Infrared and Raman spectra of N-di-isopropylphosphorylguanidine (DPG)

C.A. Te´llez, S et al. / Spectrochimica Acta Part A 56 (2000) 1563–1574 Table 1 Vibrational spectra of N-di-isopropylphosphorylguanidine (DPG) (cm−1)a IR (solid)

IR-BCA

3441 (35)

3449 3414

3315 3172 3126 2981 2932 2899 2888 2880

(27) (28) (21) (33) (16) (2)

1658 1646 1615 1592

(45) (47) (49) (54)

1556 sh

3313 3174 3118

1673 1659 1645 1616 1591 1575 1554

R(solid)

3424 3405 3360 3315 3183

(0.3) (0.3) (0.4) (0.9) (0.7)

2983 2938 2921 2888 2872

(6.7) (7.4) (6.6) (1.6) (1.5)

1656 1645 1626 1606 1570 1565 1550 1528

(0.6) (0.6) (0.7) (0.8) (0.7) (0.7) (0.7) (0.7)

R(aq.sln)

R-BCA

3412 3368 3311 3188 2991 2945

1630

1653 (1607) 1563

1511 1491 (1.2) 1480 (1.6) 1467 1452 1384 1373 1355

(30) (30) (30) (25) (15)

1469 1451 1384 1373 1354

1240 (27) 1196 (53) 1177 (46)

1242 1196 1177

1136 (26) 1109 (43) 1077 (27)

1138 1110

1015 (56)

1020

994 (58)

991

936 928 891 887 831

929

(7) (sh.) (36) (38) (29)

894 885 830

777 (26)

777

1452 1386 1373 1357 1341 1331 1303 1240 1191 1177 1168 1137 1107 1079 1067 1014 1007 995 971 965 932

(6.7) (1.6) (1.0) (3.4) (2.0) (2.2) (0.6) (0.7) (3.6) (2.7)

893 887 834 813 800 777

1481 1453

1455 1389

1356

1355 1333

1185

1196 1179

(3.1) (3.7) (0.9) (1.8) (1.3) (1.3) (1.5) (0.7) (0.6) (1.5)

1145 1109

1137 1108

932

933

(3.4) (3.9) (2.3) (1.0) (0.7) (1.5)

891

892 883 833

1065 1013 997 986

778

Approximate assignment n(NH)(I,II) 2983+441= 3424 n(NH)(I,II) n(NH)(I,II) n(NH)(II) n(NH)(I) n(NH)(I) nas(CH)(-CH3) nas(CH)(-CH2) ns(CH)(-CH3) ns(CH)(-CH2) n(CH) methine 936+739=1675 n(C =N)(II) n(C =N)(I) d(HNH)(I,II) d(HNH)(II) 834+751=1585 1240+326=1566 or 1191+373= 1564 1240+326= 1566 or 1191+373=1564 1014+534=1548 500+1014=1514 d(HCH)(-CH3) 995+494= 1489 or d(HCH)(-CH3) d(HCH)(-CH3) d(HCH)(-CH3) or d(HNH)(II) bonded d(HCH)(-CH3) d(HCH)(-CH3) d(HCH)(-CH3) d(HCH)(-CH3) d(CCH/HCO) d(CCH/CCO)+n(CO) d(HCH)(-CH3) n(P= O) nas(CC) ns(CC) n(C-N)(I,II) n(CN)(I,II) 740+349=1089 740+326=1066 ns(PO)+n(PN) d(CCH/CCO)+d(HCO) nas(PO) 613+373=986 554+408=962 nas(CO)+d(HCO) ns(CO) n(PN)(II) n(PN)(I) n(CC)+d(HCO/CCO) d(CNH)(I,II) d(CNH)(I,II) d(NH)(I,II)

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1568 Table 1 (Continued) IR (solid) 754 740 733 617 561

(10) (11) (9) (32) (34)

532 (26) 500 (sh.) 453 (24) 406 (18) 398 (10) 372 (10)

280 (7.0) 203 (25) 174 (20) 100 (20)

IR-BCA 754 736

R(solid) 751 740 732 613 554 547 534 494 451 423 408 388 373 349 326 312 302 275 254 198 175 135 113 55 40

(4.3) (7.9) (10.0) (1.6) (1.8) (2.1) (2.8) (1.8) (3.0) (2.4) (2.5) (1.6) (1.6) (1.4) (1.9) (2.5) (2.3) (4.4) (2.1) (2.5) (3.4) (8.1) (3.0) (0.2) (0.3)

R(aq.sln)

735

R-BCA

734

Approximate assignment n(CC)+d(HCO) d(CNH)(II) d(CNH)(I) d(O =PO/OPN) 451+113= 564 494+55= 549 d(O =PO/OPN) d(PNC)? d(H3C-C-CH3)? d(CCH/CCO)+n(CO) d(OPO)+d(O= PN) 275.3+113.1=388.4 d(CCH/CCO)+n(CC) d(O= PN)+d(OPO) d(N =CN/NCN) das(COP)? ds(COP)? d(N =CN) Torsion Torsion Torsion Torsion Torsion Lattice Lattice

a Values for the intensity of the infrared Raman spectrum are expressed in% T. For the Raman spectrum the values were scaled by the value of the band of higher intensity for which we assigned the value of ten (10).

n(CN) vibrational modes, one for each of the tautomeric structures. The CN stretching falls in the region of 1680 – 1580 cm − 1. In fact, in the IR spectrum of solid DPG we observed two large and overlapped bands between 1700 to 1550 cm − 1. Each one of these bands present doublet structure with peaks at 1658, 1646 and at 1615, 1592 cm − 1, respectively. A band contour analysis shows the presence of another overlapped bands at 1673, 1575, and at 1554 cm − 1. Our assignment for the n(CN) normal modes, confirmed by NCA for the framework of the phosphorylamine (I) and phosphorylimine (II) structures are: n(CN)(I) at 1646 (IR), 1645 (BCA), 1645 (R) cm − 1 and at 1653 (BCA) cm − 1. n(CN)(II) at 1658 (IR), 1656 (R) cm − 1 and at 1659 (BCA) cm − 1. The Raman shifts at 1645 and at 1656 cm − 1 are very weak in intensity. In the Raman spectrum of DPG in aqueous solution, the n(CN)

normal modes were not observed. Fig. 2 shows the IR and Raman spectra in the region of 1500 to 1850 cm − 1.

3.3. NH deformations Assuming a Cs symmetry for DPG, the vibrational representation for the NH deformations are: G (I)=5a% + a¦ and G (II)= 4a% + 2a¦ without redundancies. This means that we expect almost 12 bands corresponding to the vibrational modes in the both infrared and Raman spectra. According to the literature reports [16], primary amines have absorption bands between 1650 to 1590 cm − 1 in simple amines and also bands in the range of 900–650 cm − 1 due to external deformations of the -NH2 group. Stewart et al. [19], based on an extensive study of primary and secondary aliphatic amines, had assigned the NH angle deformations as follows: NH2 bending normal

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Fig. 2. Infrared and Raman spectra of DPG in the region of 3100 – 3500 cm − 1 and 1500–1850 cm − 1.

modes of liquid primary amines with primary alpha carbon absorbs at 778915 cm and at 8259 15 cm − 1; with secondary alpha carbon at 7949 16 and at 840 9 11 cm − 1; with tertiary alpha carbon at 7769 8 and at 8339 18 cm − 1. The NH bending frequencies of liquid secondary amines present only one absorption at 739911 cm − 1 for compounds with primary alpha carbon and at 728 9 18 cm − 1 for compounds with secondary alpha carbon. Stewart’s assignment was supported by a simplified normal co-ordinate treatment of the RNH2 and R2NH vibrations. Bending for the phosphorylimine (II) structure can be classified as: i) d(NH2) scissoring; ii) d(NH2) wagging; and iii) d(NH2) twisting. Two out of plane bending can be also defined for the phosphorylamine (I) structure of DPG. The trans secondary carboxamides [-CONHR] have an intense absorption in the IR spectrum near 1550 cm − 1. Nyquist et al. [18] studied the NH and ND bending vibrations for bonded (dimeric) and

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non-bonded (monomeric) forms of compounds with a OPNHR structural fragment for which they found two d(NH) frequencies, the lower one resulting from the monomeric form and the higher frequency from the hydrogen bonded form, i.e.: for O- (2,4,5-trichloro phenyl) N-ethyl phosphoramide the IR band at 1415 cm − 1 was assigned to the d(NH) monomer and the band at 1442 cm − 1 was assigned for the d(NH) bonded. In the DPG spectrum we did not observe any band at ca.1415 cm − 1. The IR band of DPG at 1467 cm − 1 with a shoulder at 1452 cm − 1, coincident with the Raman shift at 1452 cm − 1 can be assigned to the bending d(HCH) of methyl groups or to d(CNH)(I) bonded. Structures (I) and (II) have also the -NH2 group, and as the distinction between the d(HNH) bending of each structure is not simple, however our assignment for the d(CNH) (I) and d(HNH) (I, II) bending modes is supported by a normal co-ordinate treatment of the simplified HNC(NH)(NH2) (I) and NC(NH2)2 (II) structures, they must be considered as approximate. Our results indicate that the most probable assignment for the d(CNH) (I) and d(HNH) (I, II) bending are: 1615 (IR)

1620 (R)

813 (R) 800 (R) 777 (IR)

777 (R)

740 (IR)

740 (R)

733 (IR)

732 (R)

d(HNH) (I, II)1592 (IR)1606 (R) d(CNH) (I, II) d(CNH) (I, II) d(CNH) (I, II) d(CNH) (II) d(CNH) (I)

d(HNH) (II)

The observed IR bands at 1384 and at 1373 cm − 1 correlated with the Raman shifts at 1386 and 1376 cm − 1, were assigned as the characteristic bending doublet in gem-dimethyl groups.

3.4. n(PO) normal mode The PO stretching can be assigned at 1196 cm − 1

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in the IR spectrum and at 1191 cm-1 in the Raman spectrum. In aqueous solution, this band appears as a weak band at 1185 cm − 1.

3.5. PNC normal modes Literature values for the n(CN) and n(PN) ‘isolated ’normal modes were given by Thomas [17] for a series of compounds with PNHR structural fragments, where R is an alkyl group. According to Thomas [17], for the monoamidates we can expect absorption frequencies in the IR spectrum for the n(CN) in the region between 1099 and 1140 cm − 1. Stewart [19], assigned the asymmetric CN stretching of secondary amines at 113997 cm − 1 in compounds having a primary alpha carbon and at 1181910 cm − 1 in compounds having a secondary alpha carbon. The primary amines have a corresponding CN band at 10799 11, 10409 3 and 103098 cm − 1 for compounds with primary, secondary and tertiary alpha carbon, respectively. Chittenden and Thomas [20] in his study on characteristic infrared absorption frequencies of organophosphorus compounds, had discussed and suggested that the assignments for the PN bond are best identified by indirect correlation described for PNH2, PNHR, PNR2, PN (CH2)2, PNC and PNP groups, and as a results of these correlation it was concluded that the n(PN) should be assigned to the range 873 – 1053 cm − 1. For monoamidates with PO bond the vibration frequencies of PNH2 group is between 922 – 1057 cm − 1, and for diamidates with PO bond the region of frequencies for the PN stretching is between 943 – 1000 cm − 1. For the PNHR (alkyl) group the vibrational frequencies for monoamidates and diamidates with PO bond are in the region of 873 – 1040 cm − 1 and 815–1053 cm − 1, respectively. For the n(PN) vibrational modes, the expected region is between 873 – 1040 cm − 1. In DPG if tautomerism equilibrium between the (I) and (II) structures exists, nitrogen atoms have different hybridization; consequently, the PN bond in both structures is not the same. Similar consideration can be done for the CN bond in structures (I) and (II). So, we expect to find different frequencies or Raman shifts in the IR and Raman spectra for the n(PN) and n(CN) vibrational modes.

In DPG, the IR band at 1136 cm − 1 and the Raman shift at 1137 cm − 1 (1145 cm − 1 in aqueous solution) were assigned to the n(CN) of the phosphorylimine and phosphorylamine structures. The IR band at 1109 cm − 1 with the corresponding Raman shift at 1107 cm − 1 in the solid state and at 1109 cm − 1 in aqueous solution was assigned to the n(CN) of both (I) and (II) structures. One band with two peaks at 892 and at 887 cm − 1 was observed in the IR spectrum, this band is correlated with the Raman shift at 893 and at 887 cm − 1. Our assignment is: at 891 (IR) and at 893 (R) for the solid sample and in aqueous solution to n(PN)(II) normal modes and the frequencies at 887(IR) and 887 (R) cm − 1 to the n(PN)(I). Details of the IR and Raman spectra between 1050 to 1300 cm − 1 are illustrated in Fig. 3.

3.6. CC normal modes In DPG structure two gem-dimethyl groups can be found with the following vibrational representation for the n(CC) stretching modes: GCC = 2a% +2a¦, this means that we expect four n(CC) stretching modes in the vibrational spectrum. For the gem-dimethyl structure, the n(CC) modes absorb in the IR spectrum between 1170 and 1145 cm − 1 [21], a region nearly coincident with the absorption of the CN stretching. The infrared and Raman bands at 1177 cm − 1 were assigned as one of the CC stretching vibrational modes. The BCA reveals an overlapped band at 1168 cm − 1, which was assigned to the symmetrical CC stretching. The other pair of CC stretching was found at 751 and at 834 cm − 1 and was assigned as the ns(CC) and nas(CC) vibrational modes, respectively.

3.7. POC normal modes The vibrational representation for the n(PO) and n(OC) normal modes in DPG is: GPO= a%+a¦ and GOC=a%+ a¦, respectively. We expect also find one symmetrical and one antisymmetrical PO and OC stretching modes. According to Colthoup et al. [22], a very strong band occurs at 1050–979 cm − 1 in compounds that have the POC aliphatic link and this is probably due to the symmetric ns(POC) stretch-

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Fig. 3. Infrared and Raman spectra of DPG in the region of 1300 – 1550 cm − 1 and 1050 – 1300 cm − 1.

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ing. Our question is which of the vibrational modes n(PO) or n(OC) has higher participation in the coupled mode at 1050 – 979 cm − 1. Nyquist [18] claimed the existence of n(POC)2 out of phase and n(POC)2 in phase vibrational modes for a series of phosphoramides in which the frequency has a range from 1028 to 1055 cm − 1. For DPG, in the region of 1050 to 950 cm − 1 we found two very strong and broad IR bands at 1015 cm − 1 and at 994 cm − 1. Our experimental assignment for the n(PO) vibrational mode is: ns(PO), 1015 (IR) and 1014 (R) cm − 1; nas(PO), 994 (IR) and 995 (R) cm − 1. For the coupled n(CO) vibrational modes our proposal of assignment is: nas(CO), 936 (IR) and 932 (R) cm − 1 for the solid sample and in aqueous solution; and ns(CO): 928 (IR) cm − 1. In the Raman spectrum, we do not observe the correlative IR wavenumber at 928 cm − 1. Details of the IR and Raman spectra are given in Figs. 3 and 4. Table 1 presents the vibrational frequencies of the phosphorylimine (II) and phosphorylamine (I) structures.

3.8. Normal co-ordinate analysis (NCA) The aim of this paper was to analyze the possibility of tautomerism of N-di-isopropylphosphorylguanidine and not to assign the 3n −6= 90 vibrational modes of an asymmetric molecule. The NCA was carried out based on local symmetry of different fragments, in the same way as in di-i-propoxyphosphorylbenzylisothiourea [23], with the purpose to assist in the assignment of the more representative wavenumbers of functional groups and chemical bonds in the molecular structure. These fragments were(CH3)2CHO of Cs symmetry considering the -CH3 group as a point mass of 15 Da.O3PN of tetrahedral geometry of Cs symmetry.HNC(NH)(NH2) (I) and NC(NH2)2 (II) both approximate to Cs symmetry. In all cases the least square refinement were carried out using the general valence force field (GVFF) model [24]. Geometrical parameters were taken from reference [25]. Results of the assignment are given in Table 1.

4. Conclusions As it is shown in Fig. 2, the NH stretching group frequencies in the region of 3500– 3050 cm − 1 is an indicative for DPG of the presence of tautomerism between the phosphorylamine (I) and the N-phosphorylimine (II) structures. The overlapped bands at 3449 (IR, BCA) and 3368 (R, BCA) cm − 1 follows the Bellamy and Williams relation [16,26]: n(a%)= 345.5+ 0.876 n(a¦). Frequencies in the region of 3449–3360 cm − 1 were assigned to the NH stretching of the -NH2 groups of the phosphorylimine (II) and phosphorylamine (I) structures. The structure (II) does not present -NH groups, then by exclusion the vibrational frequencies in the range of 3300 to 3113 cm − 1 were assigned as to belonging to the phosphorylimine (I) structure. The expected region for the absorption of the n(CN) vibrational modes of free guanidine is at ca. 1660 cm − 1 [27]. The contour of the IR band which presents a doublet structure with two peaks at 1646 and at 1658 cm − 1, was assigned to the n(C= N) vibrational modes of the two tautomeric structures of DPG. Raman shifts at 1645 and at 1656 cm − 1 are very weak in intensity. The bending vibrational modes d(HNH) and d(CNH) of the two tautomeric structures were assigned in the region of 1630 to 1600 cm − 1. The BCA of the IR band at ca. 1600 cm − 1 shows the presence of five overlapped bands which can be assigned to the d(HNH) and d(CNH) bending of the tautomeric structures of DPG. Other d(CNH) bending were found and assigned in the region of 820–730 cm − 1. The different n(CN) and n(PN) vibrational modes were assigned at:n(CN) (I, II): 1136 (IR), 1137 (R), and 1145 cm − 1 (R)aqueous solution. n(CN) (I, II): 1109 (IR), 1107 (R), and 1109cm − 1 (R)aqueous solution. n(PN)(II): 891 (IR), 893 (R), and 891 cm − 1 (R)aqueous solution. n(PN)(I): 887 (IR), 887 (R) cm − 1. The symmetric and asymmetric n(PO) and n(CO) normal modes were also characterized in the IR and Raman spectra.

C.A. Te´llez, S et al. / Spectrochimica Acta Part A 56 (2000) 1563–1574

Fig. 4. Infrared and Raman spectra of DPG in the region of 920 – 1040 cm − 1 and 720 – 920 cm − 1.

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C.A. Te´llez, S et al. / Spectrochimica Acta Part A 56 (2000) 1563–1574

The assignment of the frequencies presented here must be considered as an experimental approach based on the IR absorption of characteristic frequencies. A modified normal co-ordinate analysis, based on Cs local symmetry of several fragments of the molecular structure, was very useful to assist in the assignment of the most characteristic wavenumbers.

Acknowledgements Claudio Te´llez thanks the financial support of FINEP/PADCT (project No. 65.953810) and to CNPq for the research grant.

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