Solvent effects upon the NH vibrational frequencies of phosphoramides

Spectrochimica Acta, Vol. 38A, No. 3, pp. 351-356, 19~2 Printed in Great Britain

0584-8539/82/030351--06503.00/0 © 1982 Pergamon Press Ltd

Solvent effects upon the N-H vibrational frequencies of phosphoramides FERNANDO IBA~EZ W. Instituto de Ciencias Quimicas, Pontificia Universidad Cat61ica de Chile, Santiago de Chile and JOSEFINA PEIqA A. Facultad de Ciencias, Universidad de Chile, Santiago de Chile

(Received 26 February 1981; revised 24 August 1981) Abstract--The effect of various solvents upon the N-H vibrational frequencies of the phosphoryl trisanilide (PTA), thiophosphoryl trisanilide (TTA), phosphoryl triscyclohexylamide (PTC) and thiophosphoryl triscyclohexylamide (TTC) was measured. The solvent shifts for the different amides are correlated and there is also some correlation between solvent shifts and Gutmann's solvent donicities. For the calculation of the solvent shifts a better linear relation was obtained through an equation of three terms which takes into account the donicity, the acceptor number, the dipole moment and the relative permittivity of the solvent. The last term considers the interaction of the solvent dipole moment with the induced dipole on the N-H bond. These correlations are discussed. INTRODUCTION

Several papers have reported experimental studies of solvent shifts for the N - H i.r. absorption bands of amines [1] and carboxylic amides [2], but only a limited study of this type was done on phosphoric amides [3], in spite of their importance as biologically active substances [4]. In this work we report the i.r. spectra in solution of the following four structurally analogous phosphoric oxyamides and thioamides. OP(NHC6Hs)3

(PTA)

SP(NHH6Hs)3

(TTA)

OP(NHC6Hll)3

(PTC)

SP(NHC6HI03

(TTC)

The structural features of these amides are important for the study of solute--solvent interactions because they have the phosphoryl or thiophosphoryl group for the same R(phenyl or cyclohexyl). On the other hand they have an aromatic or an aliphatic group attached to the nitrogen atom. The earliest theoretical dielectric approach to the i.r. frequencies shifts was due to KIRKWOOD[5], BAUER and MAGAT[6]; later BUCKINGHAM[7] and PULLIN [8] developed a more rigorous treatment. But these theories have limited success in problems involving specific interactions such as hydrogen bonding. In spite of this fact the solvent dipole moment plays an important role on the i.r. solvent shifts. It is known that many compounds, amines for example, show abnormally high values of the ap351

parent dipole moments in several electron donor solvents [9--11]. For hydrogen-bonded complexes in "inert" solvents there is also a relation between the enthalpy of formation of the complex AH and the dipole moment parameter A/z1/2[12]. A/~ is the vectorial difference between the dipole moment of the complexed system and that of the individual components. We developed an empirical equation for the calculation of the solvent shifts. This equation contains the solvent parameters of the donor number or donicity[13] and the acceptor number[14], but also a term which considers the interaction of the solvent dipole moment with the induced dipole[15] on the N - H bond of the phosphoramides. This last term provides a considerable improvement of the least-squares correlation for the calculated frequencies vs their experimental values. EXPERIMENTAL

The solvents Merck UVASOL and Merck p.a. were dried and stored over type 3/~ or 4/~ molecular sieves in dark bottles. PTA, PTC and TTC were prepared from the reaction of phosphoryl or thiophosphoryl chloride and the amine following reported methods in the literature[16]. TTA was synthesized by refluxing P4SI0 with aniline[17]. The products were recrystallized several times from absolute ethanol (PTA, TTA, TTC) or n-hexane (PTC). The melting points and the i.r. spectra of the solids showed that the products were pure and anhydrous. I.r. spectra were obtained with Perkin-Elmer 700 and Perkin-Elmer 621 spectrometers. Good agreement was found between the two instruments. The concentration of the solute was 0.005 mole fraction. In all cases compensation for any solvent absorption was arranged by using a second cell in the blank beam. In the solid state the spectra were taken using mulls in Nujol between KBr plates.

352

F. IBAI;IEZW. and J. PE~A A. RESULTS AND DISCUSSION

Table 2. N - H absorption f r e q u e n c i e s ( c m , 1)

For comparison the N-I-I vibrational frequencies of the solid amides are given in Table 1. TTA and TTC show two N-H absorption bands, which have nearly identical shape and frequencies in both compounds. The band at 3360 cm -1 is narrow and corresponds to the free N-H bonds whereas the band at 3250 cm -1 is broad and can be assigned to the intermolecularly b o n d e d / ~ N - H . . . . N ~ . For the solid phosphoric thioamides no influence of the nature of the R group upon the i.r. N - H absorption frequencies could be detected. Interactions of the type N - H . • • 7r in the TTA must be therefore very weak or non-existent. The N-H absorption bands of the solids PTA and PTC are broader than the corresponding bands of the thioamides. In all the solid amides the band of lower frequency at 3250 or 3300 can be assigned to the ~/N-H . . . . N /

interaction. But in the phos-

phoryl amides this band can also arise from the

1 2

Solvent

DN (a)

TTA

PTA

TTC

PTC

1,2-Dichloroethane Benzene

.... 0.1

5579 5578

3384 5396

3592 3400

5415 3410

0.7 2.7 4.4 14,1 14.8 16.5 17.1

5578 3568 3362 3311 3258 3319 3317

(c) 5373 3370 3324 3328 3327

5380 (b) 3382 5370 3320 3350 3360

5585 (b) 3382 (b) 3340 3342 (b)

19.2

3225 3236 3200

3208 3250 3212

3212 3306 3270

(b) 3294 3290

3 Acetyl chloride 4 Nitromethane S Nitrobenzene 6 Acetonitrile 7 Dioxane 8 Methyl a c e t a t e 9 Ethyl a c e t a t e 10 11 12

13 14

Diethyl ether Tetrahydro~uran

20.0

Tributylphosphate Chloroform Carbon disulfide

23.7

.... ....

3398 3392

3272

3410 (b)

3415 3390

3415 5313

aFrom Ref. [13]. bSparingly soluble. CReacts.

in which all the measurements were possible (solvents 1, 2, 5, 7, 8, 11, 12, 13), because otherwise no comparison can be attempted. Figure 1 shows a typical plot of the frequencies for two pairs of compounds and the least-squares line in each case. The magnitude of the solute-solvent interactions for the pairs T T A - P T A and T T C - P T C seems to

~/N-H . . . . O=-P~ interaction. In the i.r. spectra of the solutions we observe only one N - H absorption band. In solution the

Table 3. Linear correlations of phosphoramide N-H frequencies

interactions of the type-N/N-H . . . solvent pre-

Compounds

dominate because the N - H frequency shifts are directly related to the solvent donicity. Dilution has no effect upon the solvent shifts. The N-H absorption frequencies in several solvents and the DN solvent donicities [13] are given in Table 2. The N-H absorption frequencies are abnormally high in chloroform, which has a very high acceptor number[14]. The reason for this may be a relat!vely strong interaction between the proton of the chloroform and the unshared electron pair of the nitrogen in the amides. BELLAMY and WILLIAMS have shown[18] that the Vco frequencies of two carbonyl compounds measured in several solvents are linearly correlated when plotted against each other. There are reasonably linear relationships between the N-H absorption frequencies of the amides. The slopes (m), the intercepts (b), the standard deviation (S.D.) and the correlation coefficients (r) for the least-squares line for 8 data points are shown in Table 3. We have considered here only the eight solvents

TTA-PTA TTA-TTC TTA-PTC PTA-TTC PTA-PTC TTC-PTC

N-H

absorption 3380

PTA TTA PTC TTC

5480

3430

b(cm "2 )

0.986 0.679 0.667 0.689 0.676 0.976

56 1103 1150 1061 1112 86

r

SD(cm "l )

0.999 0.994 0.966 0.997 0.967 0.966

12 6 17 4 19 17

0

o

//o

Jo / •

o//// /././o

/

/"

1"/'7"

/

32BI /

Table 1. Compound

m

frequencies (cm -l) 3350

3300

3358

3250

3370

3250

3360

3250

]2

t)TTA Fig. I. Plots of the N-I-I vibrational frequencies(cm-j) for PTA ( , Q) and TTC (. . . . . . , O) vs those for TTA.

Solvent effects upon the N-H vibrational frequencies of phosphoramides be nearly independent of the chalcogen atom bound at the phosphorus since the slope of both lines are close to unity. We conclude that the interactions - ~ P = O . . .

solvent and ) P = S . . .

solvent for these pairs of compounds are almost negligible compared with the interaction ) N H . . . solvent. The ) N - H • • • solvent interactions of the trisanilides are considerably stronger (m = 0.68) than that for the trfs-cyclohexylamides. In these pairs of compounds the steric and specially the electronic factors are very different and the presence of an aromatic ring in the PTA or TTA allows a delocalization of the free electron pair at the nitrogen, hence the N - H group becomes to a great extent more acidic in these compounds according to the resonance structures for the NHPh group (Scheme 1). When all the experimental points are plotted an important deviation of the points corresponding to the carbon disulfide arc observed only for the compound pairs TTC-PTC and TTA-PTC. In this solvent the N - H frequencies for the thiophosphoryl compounds are larger than expected from the linearity. An specific interaction between the P=S group and the solvent may be responsible for this fact. The extrapolation of the least-squares line of a plot of the N - H frequencies for each amide vs the donicities of the solvents allows the calculation of the frequencies at zero donicity, 1,o, for each compound. The N - H absorption band becomes narrower and appears at higher frequencies when the solvent donicity decreases. The slopes (m), the intercepts (vo), the correlation coefficients (r) and

H

353

the standard deviation (S.D.) for n data points are given in Table 4. A plot of the N-H frequencies for each amide vs the donicities of the solvents and the data of Table 4 shows that the correlation coefficients and the standard deviation, specially for the TTC are far from satisfactory, but we cannot compare them because some measurements were not possible in several cases (see Table 2). In the six solvents in which the donicities are known and all measurements are possible (solvents 2, 5, 7, 8, I 1 and 12) the correlation coefficients for the four amides are comparable. There is no indication that these phosphoryl amides are associated as found by B. N. LASKORIN et al. in their study[3]. These authors measured the i.r. spectra of solutions of phosphoramidates (R'O)nP(O)(NHR)3_, (in which n = 0, 1, 2; R and R' = alkyd in four solvents: acetonitrile, toluene, carbon tetrachloride and cyclohexane. They conclude the existence of an intermolecular hydrogen bond of the one single type ~N-H...O=P-~.

The

deviations

H

H

H

Scheme I.

Table 4. Correlation of N-H vibrational frequencies (cm-1) with solvent donicities In a l l m

vo

the

linearity cannot be explained here by the existence of strong self-association. We searched therefore for an explanation of the observed deviations. It is known that the donicities give bad correlations when steric factors come into play, as has been observed in the interactions of HMPA and (CH3)3NO with various metal ions[19,20]. In all the four amides the steric factors are important, but the deviations from the linearity are, in spite of this fact, too pronounced. We used a function which contains both the donor number and the acceptor number of the solvent, since the acidic and basic properties of the solvent are of importance in hydrogen bonding[21]. We

©

Compound

from

solvents

In g r o u p

-r

SD

n

m

vo

of

common

solvents

-r

SD

n

PTA

-7.37

3404

0.881

30

10

-7.38

3402

0.940

23

6

TTA

-6.99

3389

0.911

26

11

-7.29

3389

0.939

24

6

PTC

-4.64

3402

0.968

11

7

-5.06

3410

0.977

10

6

TTC

-5.25

3404

0.747

37

10

-5.06

3405

0.9S0

14

6

F. IBA~z W. and J. PEI~AA.

354

obtained for the solvents 2, 4, 5, 6, 7, 10 and 11, for wlfi'ch both, donicities and acceptor numbers are known, the least-squares regression equations of vN_n (cm -I) on D N (Eqns la, lb, lc) and of VN_H on DN and A N (Eqns 2a, 2b, 2c)

The long-range energy of interaction between two molecules having permanent electric moments can be considered as being made UP of the sum of three terms [22, 15] E ~----ECS "Jr-E i n d ")t-E d i s

v,(PTA) = 3402 - 8.26 DN v2(PTA) = 3360 - 7.38 DN + 2.68 A N

(la) (2a)

v,(TTA) = 3389 - 7.77 DN v2(TTA) -- 3 3 5 4 - 7.03 DN + 2.24 A N

(lb) (2b)

vI(TTC) -- 3 4 1 4 - 6.76 DN v2(TTC) -- 3331 - 5.69 DN + 6.41 AN.

(lc) (2c)

Only six solvents can be considered in the case of TTC (2, 5, 6, 7, 10 and 11). As shown in Table 5 the least-squares lines for the calculated frequencies vs their experimental values are substantially improved, for the P T A from 0.950 to 0.971, for the TTA from 0.967 to 0.986 and for the TTC from 0.796 to 0.927. PTC was excluded here since there are only four solvents in which all parameters are known. For all the amides the introduction of the acceptor number as variable for the calculation of the N - H vibrational frequency is very important especially in the case of acetonitrile and diethyl ether, which have a large (18.9) and a small (3.9) AN-value, respectively. In the first solvent the calculated frequency through Eqn (1) is too small, whereas for the second solvent it is too large. We feel that the calculated N - H vibrational frequencies could be improved further by considering the effect of the dipole moment of the solvent upon the N - H bond.

(4)

The first term represents the purely electrostatic interaction between the permanent charge distributions of the two molecules. The second term, Eind, is the Debye-Falkenhagen energy of induction and represents the energy of the interactions between the permanent charge distribution of one molecule and the moments induced in the other molecule. The last term, Edi,, is the London dispersion energy and represents the interaction between the two induced charge distributions. We assume that the second term l~.nd is represented unsatisfactorily in Eqn. (2). It is known that the term varying as 1/r 6 is the major term in the interaction between neutral molecules and dipole moments. We obtained in fact a considerable improvement of the correlations by considering the interactions of the dipole moment of the solvent with the induced dipole on the N - H bond through Eqn. (3), where /z is in Debye units and e, is the relative permittivity of the solvent Av = a DN + b A N + ctz21e 2.

(3)

The linear regression equations of v on DN, A N and/22/~ 2, are given in Eqns (3a-3c) v3(PTA) = 3410 - 3.30 DN - 0.593 A N

(3a)

- 1905/~21e2

Table 5. Calculated frequencies (cm-I), correlation coefficients (r) and standard deviations (S.D.) for the regression Eqns (1), (2) and (3) PTA

~LVENT

~(a)

Benzene (2)

8.2

~(D)

0

Cr

Ve~.

v:

TTA

v2

v3

Ve~"

TTC

~:

v2

v3 Ve~"

~1

2.3

3396

3401

3381

3405

3378

3388

3372

3381 3 4 0 0 .

.

oz

3413 .

.

.

v3

3383 .

.

3419

Nitrom~thane (4)

20.5

3.46

35.9

3373

3380

3395

3371

3368

3368

3381

3371

.

Nitrobenze-

14.8

4.28

34.8

3370

3366

3367

3388

3362

3355

3356

3355 3382

3384

3401

3359

18.9

3.97

38.0

3324

3286

3307

3331

3311

3279

3297

3307 3370

3319

3372

3392

10.8

0.46

2.2

3272

3280

3280

3272

3258

3274

327~

3270 3320

3314

3316

3296

3.9

1.17

4.3

3208

3243

3229

3203

3225

3240

3228

3217 3212

3284

3247

3216

8.0

1.63

7.6

3250

3237

3234

3252

3236

3234

3231

3238 3306

3279

3268

3310

1.000

0.950

0.971

0.995

1.000 "0.967

0.986

0.796 0.927

0.963

0

21

16

7

ne (5) Acetonitril

(63 Dioxane

(7)

Diethylether (10) Tetrahydro-

furan (11) r SD (cm- I )

aFrom Ref. [14].

0

16

11

0.994 1.000 7

0

39

24

19

Solvent effects upon the N-H vibrational frequencies of phosphoramides ~(TTA) = 3 3 7 4 - 5.36 DN + 0.913 AN

(3b)

Oexp.

- 779~2/e 2 v3(TTC) = 3440 + 2.93 DN - 2.63 AN _ 3644/~2/e2"

3~o

//exp ---- //calc.

~

32R

In Eqns (3a), (3b) and (3c) the constant term can be considered as the vibrational frequency of the N - H bond in the dilute gas phase. The second term contains the donor number, which is an enthalpy of association. If the volume changes in the liquid phase can be neglected, this term is also essentially an interaction energy term. In the third term the acceptor number is related to the ~'PNMR shifts produced in triethylphosphine oxide by the electrophilic solvent actions[14] which lower the electron densities at the P-atom due to

l)exp.

o

325(

J

Oc)

The correlation coefficients and the standard deviations for the plots of the calculated frequencies vs their experimental values are shown in Table 5. Figs 2, 3, 4 show plots of vc~c vs vcxp for PTA, TTA and TTC. For clarity reasons the leastsquares lines are not shown, but only the line

355

~

o

0colc. Fig. 2. Plot of the calculated frequencies vs their experimental values for PTA using Eqns (la) (O) and (3a) (O).

/ S A "qt"'--'-~ 0 ~

3~o

~oo fiso Ocalc. Fig. 3. Plot of the calculated frequencies vs their experimental values for TTA using Eqns (lb) (O) and (3b) (@).

inductive effects [23]. Close relationships exist between the acceptor number and the free energies of solvation of halide ions[14]. If the entropy effects are negligible or at least nearly constant in our experiments, and in the experiments in which the acceptor number has been determined, this third term can also be considered as an interaction energy term. The last argument is important, because in our case we have also phosphoryl or the closely related thiophosphoryl compound instead of (Et)3PO. In the last term/~2/2 is proportional to the DEBYE interaction energy between the dipole of the solvent close to the substrate and the induced dipole on this last molecule. In a very crude approximation following GUTMANN[23] we can represent the substrate as structure I (Scheme 2) We conclude that in the interactions responsible for the frequency shifts between the N - H group and the different solvents there are forces caused by the induced N - H bond polarization. It is interesting to discuss the relevance of the last term in Eqn. (3). The energy of attraction resulting from the distortion of the electron cloud of the substrate molecule by the permanent moment of the solvent molecule is proportional to

R

SA: solvent acceptor function

p~ ' - /

H

dso

~"SD Scheme 2.

SD" solvent donor function

356

F. IBAI~IEZW. and J. PEI~A A. 0exp.

3t,0C

o

33~

not through another solvent molecule. This fact is also consistent with the study of KOLLMAN and ALLEN[25]. These authors conclude that the distance X . . . Y for the hydrogen bond X-H •. • Y is essentially a function of the degree of positive charge of the hydrogen in the H bond; R(NH . . . Y) is approximately 3, 4/~ no matter what donor atom Y is. Our findings confirm furthermore the high specific and directional character of the hydrogen bonding in all these solvents. RIgFIgRENCE8

33O(

O

I

3250

I

I

3300

I

3350

3400

,

0cole. Fig. 4. Plot of the calculated frequencies vs their experimental values for T T C using Eqns (Ic) (O) and (3c) (0). 2 p. 2 aole,r 6,. we do not consider here the energy

term due to the interaction between the N - H dipole bond and the induced dipole on the solvent molecule. The polarizability of the substrate ao is in our case the N - H bond polarizability and is constant for each compound. What about the factor l/r6? r represents the distance between the dipoles and we can attempt to estimate r using the molar refractio~ for the sodium D-line, RD, according to the Lorentz-Lorentz equation (Eqn. 5) _n~-lM

i~, - ~

(5)

-~.

Because RD is roughly proportional to the molecular volume[24], R g 3 must be also roughly proportional to the molecular diameter. Equation (6) 2

Av = a D N + b A N + C Rt~o "

(6)

gives approximately the same correlation coefficient as Eqn. (2). For example, for TTA r = 0.983 and we conclude that the introduction of r in the last term has no physical sense at all. In other words, the interaction of the solvent dipole moment takes place close to the N - H bond and

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