Experimental and theoretical vibrational study of methylene bis(thiocyanate), CH2(SCN)2. A...

JOURNAL OF RAMAN SPECTROSCOPY, VOL. 28, 501È509 (1997)

Experimental and Theoretical Vibrational Studiesof the Amino Acid L-Asparagine in Solution

J. T. Lo� pez Navarrete, J. Casado, V. Herna� ndez and F. J. Ram•� rez*Departamento de Qu•�mica F•� sica, Facultad de Ciencias, Universidad de Ma� laga, 29071-Ma� laga, Spain

Raman spectroscopy and the self-consistent reaction Ðeld (SCRF) theory were used to study structural and vibra-tional features of the amino acid L-asparagine in and solutions. Ab initio methodology at the RHF/6–H

2O D

2O

31 + G* level were used to evaluate the minimum energy structure, quadratic force Ðeld, vibrational wavenumbersand infrared intensities of this molecule in a polar medium. Raman spectra from aqueous solutions of asparagineand its and deuterated isotopomers were recorded and a general assignment of the measured bandsND

3‘ ND

2was proposed on the basis of the isotopic shifts. The results were successfully supported by a scaled 6–31 + G*/SCRF quantum-mechanical force Ðeld. 1997 John Wiley & Sons, Ltd.(

J. Raman Spectrosc. 28, 501È509 (1997)No. of Figures : 4 No. of Tables : 5 No. of References : 38

INTRODUCTION

From a structural point of view, amino acids can beconsidered as bifunctional compounds. It is well estab-lished that the neutral form, isH2NÈCHRÈCOOH,the stablest structure for an isolated amino acid mol-ecule,1h5 that is to say, in the gas phase ; however, theyhave a zwitterionic structure, in`H3NÈCHRÈCOO~,the solid state and in polar media. Biological systemsare usually associated with aqueous solutions, wheresoluteÈsolvent interactions signiÐcantly inÑuence theenergy, structure and vibrations of these molecules.

Since Raman spectroscopy was discovered, it hasalways been a powerful tool to study experimentallysolvation e†ects in biological molecules because water isa weak Raman scatterer.6 Hydrogen bonds stabilizingzwitterionic structures of amino acids originate indica-tive wavenumber shifts in the most characteristic vibra-tional bands which can be easily measured by Ramanspectrometry. The quantum-chemical treatment of sol-vated molecules can be performed in two di†erent ways.One is based on building a “supermoleculeÏ surroundingthe solute molecule by a Ðnite number of solvent mol-ecules.7h9 This method is advantageous for studyingsome speciÐc interactions in depth, but it does notdescribe the global e†ect of the environment around themolecule. To take into account this e†ect we need toconsider a continuum dielectric approach. In thismodel, also named self-consistent reaction Ðeld (SCRF)theory, the solute electric Ðeld polarizes the surroundingdielectric medium, thus producing a reaction Ðeld thatin turn interacts with the polar solute.10h13 This modelhas been implemented to be used with semi-empirical,ab initio and density functional (DFT) methodology.Among them, that proposed by Wong et al.14,15 hasbeen successfully applied in recent years, and it has beenimplemented in the Gaussian series of programs.16

* Correspondence to : F. J. Ram•� rez.

In a series of previous works, we studied the vibra-tional spectra of amino acids containing carboxylate orcarboxamide groups.17h22 These studies were largelybased on the analysis of infrared and Raman spectra ofthe molecules as solid samples assigned with the aid ofsemi-empirical normal coordinate calculations. Thesestudies gave reliable interpretations of the vibrationalspectra of the aforementioned amino acids. However,calculations were carried out without considering anyintermolecular interaction, so that the force constantswere evaluated in a local minimum of the potentialenergy surface where isolated amino acids had zwitter-ionic structures. Recent computer advances let us inves-tigate these biological systems by using more realistictheoretical models, such as the SCRF theory, in theframework of high-level ab initio formalisms.

In this work, we studied the amino acid L-asparagine,2-amino-4-amidosuccinic acid, `NH3ÈCH(CO2~)È

in aqueous solution from both struc-CH2ÈCONH2 ,tural and vibrational points of view. This molecule canbe found as a protein component or in free form, and itplays an essential role in the metabolic control of cellfunctions in nerve and brain tissue ; it is also an impor-tant nitrogen reserve substance.23 A vibrational study ofthis amino acid in the solid state has recently beenpublished,21,22 involving isotopic labelling and normalcoordinate analysis by the MNDO semi-empiricalmethod,24 but we have not found in the literature anyvibrational studies of L-asparagine in aqueous solution.Here we report Raman spectra of this amino acid in

and solutions ; in order to support theH2O D2Oassignments, we also performed a force Ðeld calculationby using the Onsager reaction Ðeld theory. Ab initiobasis sets were used in these calculations at the 6È31 ] G* level,25 which contains polarization and di†usefunctions on heavy atoms. This basis set has been sug-gested by several workers10,26 as suitable for structuralpredictions of zwitterionic species, such as amino acids.The use of di†use functions is, in this case, advisableowing to the presence of charged groups at the periph-ery of the molecule.

CCC 0377È0486/97/070501È09 $17.50 Received 10 October 1996( 1997 by John Wiley & Sons, Ltd. Accepted 14 February 1997

502 J. T. LO� PEZ NAVARRETE ET AL .

The paper is organized as follows : Ðrst we give theexperimental details and the methods of calculations.Then we discuss the results of ab initioÈSCRF calcu-lations in relation to the structure of L-asparagine insolution. In the next section we report the Ramanspectra of this amino acid and its deuter-ND3`, ND2ated derivative, proposing an assignment for the mea-sured bands, followed by the results from a normalcoordinate calculation in the light of the reaction Ðeldtheory. In the last section we present our conclusions.

EXPERIMENTAL AND COMPUTATIONALMETHODS

We used L-asparagine (99% purity) from SigmaÈAldrichas received. Deuteration was carried out as describedelsewhere.21 Aqueous solutions were prepared by usingtriply distilled water and deuterium oxide (99.9 atom%D, Aldrich). Raman spectra of the solutions at roomtemperature were recorded on a Jobin Yvon RamanorU1000 spectrometer. Excitation radiation of 514.5 nmgenerated by a Spectra-Physics argon ion laser wasused. The best resolution obtained was 1 cm~1. Infraredspectra at room temperature between 1700 and 1100cm~1 were recorded on a PerkinÈElmer Model 1760 XFourier transform infrared spectrometer purged withdry argon. Demountable cells with calcium Ñuoridewindows were used to record the spectra in aqueoussolution. A mean of 20 scans with 2 cm~1 spectralresolution were accumulated.

Geometry optimization of the asparagine moleculewas performed at the restricted HartreeÈFock (RHF)level of theory using 6È31 ] G* and 3È21G* basissets.25 We used the Gaussian-92 program16 to generatethe SCRF-optimized structure of this amino acid as azwitterion. The SCRF formalism places the solute mol-ecules in spherical cavities of radius This is the onlya0 .adjustable parameter in such a calculation, in additionto the dielectric constant of the solvent. In the presentwork it was estimated from the solute moleculardensity, by mean of the following equation :dM ,

a03\ 3M4ndMNA

(1)

where M is the molecular weight and is AvogadroÏsNAnumber. The value obtained from Eqn (1) was increasedby 0.5 in order to account for the van der Waals radiiÓof the surrounding solvent molecules.14 The cavityradius obtained for asparagine was a0\ 3.74 Ó.

Cartesian force constants were evaluated within thesame theoretical scheme as used for the geometry opti-mizations. Second derivatives of the molecular energywere computed analytically. All the ab initio calcu-lations were performed on a Convex 240 computer atthe CICA Computer Center (Sevilla, Spain). Maximumforces (in atomic units) after geometry optimizationwere smaller than 8.9] 10~5. Maximum deviationsfrom ideally zero translation and rotation wavenumberswere smaller than 2.1 cm~1. Wavenumbers wereobtained by diagonalization of a mass-weighted Carte-sian force constant matrix. Infrared absorption inten-sities were evaluated from atomic polar tensors.27

The cartesian force constants were transformed into aset of non-redundant local symmetrical internal coordi-nates, deÐned according to the Pulay methodology.28This allows for a more useful description of the vibra-tional potential energy and makes further calculationseasier. The force Ðeld in internal coordinates was subse-quently scaled to compensate for systematic over-estimations. Wavenumbers and normal coordinateswere calculated by the Wilson FG matrix method.29

RESULTS AND DISCUSSION

Structural features

Table 1 gives the results concerning bond distances andangles for the optimized structure of asparagine mol-ecule, which are depicted in Fig. 1, where two intramol-ecular interactions are described in addition to thesoluteÈsolvent e†ects. They involve the groupNH3`and in turn the and the CxO (amide) groups. AsCO2~we will discuss later, two non-bonded HÉ É ÉO distancesare within the hydrogen bond range.30 However, therelated NÈHÉ É ÉO angles are in some cases far from lin-earity, so they can be better classiÐed as short van derWaals intramolecular contacts. The optimized confor-mation exhibits a moderate intramolecularO14É É ÉH9contact (1.99 and a second weaker interaction (2.10Ó)

involving the and atoms. These interatomicÓ) O6 H10distances are listed in Table 1, which also includes theNÈHÉ É ÉO angles and NÈHÉ É ÉOÈC dihedral angles.The 6È31 ] G*/SCRF structure deviates signiÐcantlyfrom the NÈHÉ É ÉOÈC planarity (20.7 and 40.0¡). Thereason for this can be found in the soluteÈsolvent inter-actions ; as can be seen in Fig. 1, the conformation ofthe amide group allows for a more intense electrostaticinteraction between amide hydrogen atoms and solventmolecules.

In order to compare the optimized geometry withexperimental measurements, we have included in Table1 structural parameters reported for monohydratedasparagine from x-ray di†raction techniques.31 Thedata show good agreement between the calculated andexperimental values. The mean square deviations are0.045 for bond lengths and 2.48¡ for bond angles. AsÓa general trend, the CÈH, NÈH and CÈC distancesare overestimated whereas the CÈN and CÈO dis-tances are underestimated. We should point out thatthe deviations decrease on going from the carboxylate,where the intramolecular contacts are stronger, to theamide end of the molecule, which is less hinded andbetter solvated by the surroundings. In spite of the factthat the experimental values come from solid mono-hydrated asparagine, good agreement also exists for thedihedral angles, thus indicating some analogies betweenthe two environments. On this basis, we could expectfurther correlations regarding vibrational wavenumbers.

Vibrational analysis and Raman spectra

The optimized geometry of solvated asparagine wasused as the starting point to evaluate the completequadratic force Ðeld at the 6È31 ] G* ab initio level.

( 1997 John Wiley & Sons, Ltd. J. Raman Spectrosc., Vol. 28, 501È509 (1997)

VIBRATIONAL STUDIES OF L-ASPARAGINE IN SOLUTION 503

Table 1. SCRF/6–31 + G* and experimentala geometries for asparagine moleculeb

Bond Calc. (Ó) Exp. (Ó) Angle Calc. (¡) Exp. (¡) Dihedral angle Calc. (¡) Exp. (¡)

C1—C

21.564 1.518 C

2—C

1—O

5116.2 118.2 O

5—C

1—C

2—C

3É37.2 É47.5

C1—O

51.227 1.247 C

2—C

1—O

6114.1 115.7 O

5—C

1—C

2—H

783.8 74.7

C1—O

61.248 1.268 C

1—C

2—C

3118.7 115.7 O

5—C

1—C

2—N

8É163.6 É174.9

C2—C

31.530 1.511 C

1—C

2—H

7105.5 104.4 O

6—C

1—C

2—C

3147.7 136.6

C2—H

71.083 0.973 C

1—C

2—N

8107.5 110.8 O

6—C

1—C

2—H

7É91.3 É101.2

C2—N

81.501 1.501 C

3—C

2—H

7107.8 110.9 O

6—C

1—C

2—N

821.3 9.2

C3—C

41.523 1.516 H

7—C

2—N

8105.7 103.1 C

1—C

2—C

3—C

4É77.81 É53.8

C3—H

121.083 1.006 C

2—C

3—C

4113.7 112.5 C

1—C

2—C

3—H

1242.3 65.1

C3—H

131.086 0.987 C

2—C

3—H

12107.4 105.3 C

1—C

2—C

3—H

13158.6 É172.4

C4—O

141.217 1.243 C

2—C

3—H

13110.1 106.7 C

1—C

2—N

8—H

977.6 81.6

C4—N

151.332 1.332 C

4—C

3—H

12108.5 109.2 C

1—C

2—N

8—H

10É38.1 É38.4

N8—H

91.015 0.954 C

4—C

3—H

13109.6 108.3 C

1—C

2—N

8—H

11161.0 É153.2

N8—H

101.010 0.932 C

3—C

4—O

14120.1 120.5 C

2—C

3—C

4—O

1414.9 3.1

N8—H

111.011 0.935 C

3—C

4—N

15116.4 116.5 C

2—C

3—C

4—N

15É165.4 É177.0

N15

—H16

0.998 0.929 O14

—C4—N

15123.3 122.9 H

12—C

3—C

4—O

14É104.8 É113.5

N15

—H17

0.999 0.919 C2—N

8—H

9108.2 109.5 H

12—C

3—C

4—N

1575.1 66.4

O6· · ·H

102.10 — C

2—N

8—H

10107.6 109.2 H

13—C

3—C

4—O

14138.7 120.7

O14

· · ·H9

1.99 — C2—N

8—H

11113.8 112.2 H

13—C

3—C

4—N

15É41.6 É59.3

H9—N

8—H

10107.2 109.5 O

14—C

4—N

15—H

16É167.8 É7.1

H9—N

8—H

11108.9 112.1 O

14—C

4—N

15—H

17É9.4 É175.8

H10

—N8—H

11110.5 104.1 C

3—C

4—N

15—H

1612.6 4.3

H16

—N15

—H17

115.9 117.2 C3—C

4—N

15—H

17170.9 173.0

C4—N

15—H

16122.1 119.8 N

8—H

9· · ·O

14—C

440.0 —

C4—N

15—H

17118.4 122.1 N

8—H

10· · ·O

6—C

1É20.7 —

O6· · ·H

10—N

8129.9 —

O14

· · ·H9—N

8106.9 —

a See Fig. 1 for numbering.b Data from Ref. 31.

The force constants were transformed into a set of inter-nal coordinates as listed in Table 2. We also recordedthe Raman spectra of asparagine and its ND2 , ND3`deuterated derivative in solutions of andH2O D2O,respectively (Figs 2 and 3). A general assignment wasproposed for the observed bands on the basis of a pre-

vious vibrational study on solid asparagine,21 the iso-topic shifts measured and some reported studies ofother amino acids. The CÈH stretching vibrations wereonly observed in the Raman spectrum of asparagine-d5 .In this spectrum the NÈD stretching vibrations werehidden behind solvent absorptions ; this also resulted in

Figure 1. Atomic numbering used in this work for zwitterionic asparagine.

( 1997 John Wiley & Sons Ltd. J. Raman Spectrosc., Vol. 28, 501È509 (1997)


Table 2. Symmetrized Pulay coordinates used in this work for the vibrational analysis of the asparagine molecule

Numbera Coordinateb Symbol Description

1 r1 2

l(CC) C–C stretch

2 r2 3

l(CC) C–C stretch

3 r3 4

l(CC) C–C stretch

4 2É1@2(r3 12

½r3 13

) ls(CH

2) CH

2sym. stretch

5 2É1@2(r3 12

Ér3 13

) la(CH

2) CH

2antisym. stretch

6 r4 14

l(CxO) CxO stretch

7 r4 15

l(C–NH2) C–NH

2stretch

8 r2 7

l(CH) C–H stretch

9 r2 8

l(C–NH3

½) C–NH3

½ stretch

10 2É1@2(r1 5

½r1 6

) l(C–O5) CO

2É sym. stretch

11 2É1@2(r1 5

Ér1 6

) l(C–O6) CO

2É antisym. stretch

12 3É1@2(r8 9

½r8 11

) l(N–H9) NH

3½ sym. stretch

13 6É1@2(2r8 9

Ér8 10

Ér8 11

) l(N–H10

) NH3

½ antisym. stretch

14 2É1@2(r8 10

Ér8 11

) l(N–H11

) NH3

½ antisym. stretch

15 2É1@2(r15 16

½r15 17

) ls(NH

2) NH

2sym. stretch

16 2É1@2(r15 16

Ér15 17

) la(NH

2) NH

2antisym. stretch

17 26É1@2(5b12 3 13

½b2 3 4

) d(CH2) CH

2scissor

18 26É1@2(5b2 3 4

½b12 3 13

) d(skel) CCC bend

19 1/2(b12 3 4

Éb13 3 4

½b2 3 12

Éb2 3 13

) r(CH2) CH

2rock

20 1/2(b12 3 4

½b13 3 4

Éb2 3 12

Éb2 3 13

) u(CH2) CH

2wagg

21 1/2(b12 3 4

Éb13 3 4

Éb2 3 12

½b2 3 13

) t(CH2) CH

2twist

22 6É1@2(2b5 1 6

Éb5 1 2

Éb6 1 2

) d(CO2

É) CO2

É in-plane bend

23 2É1@2(b5 1 2

Éb6 1 2

) d(CO2

É) CO2

É in-plane bend

24 /1

c(OCOÉ) OCOÉ out-of-plane bend

25 6É1@2(b11 8 10

½b10 8 9

½b9 8 11

Éb2 8 9

Éb2 8 10

Éb2 8 11

) ds(NH

3½) NH

3½ sym. bend

26 6É1@2(2b11 8 10

Éb10 8 9

Éb9 8 11

) da(NH

3½) NH

3½ antisym. bend

27 2É1@2(b10 8 9

Éb9 8 11

) da(NH

3½) NH

3½ antisym. bend

28 6É1@2(2b2 8 9

Éb2 8 10

Éb2 8 11

) r(NH3

½) NH3

½ rock

29 2É1@2(b2 8 10

Éb2 8 11

) r(NH3

½) NH3

½ rock

30 6É1@2(2b1 2 7

Éb8 2 7

Éb3 2 7

) d(CH) C–H bend

31 2É1@2(b8 2 7

Éb3 2 7

) d(CH) C–H bend

32 18É1@2(4b1 2 8

½b1 2 3

½b3 2 8

) d(skel) CCN bend

33 18É1@2(4b3 2 8

½b1 2 3

½b1 2 8

) d(skel) CCN bend

34 18É1@2(4b1 2 3

½b1 2 8

½b3 2 8

) d(skel) CCC bend

35 6É1@2(2b16 15 17

Éb4 15 17

Éb4 15 16

) d(NH2) NH

2bend

36 2É1@2(b4 15 17

Éb4 15 16

) r(NH2) NH

2rock

37 /15

c(NH2) NH

2out-of-plane bend

38 6É1@2(2b3 4 15

Éb14 4 15

Éb3 4 15

) d(CONH2) CONH

2in-plane bend

39 2É1@2(b14 4 15

Éb3 4 15

) d(CONH2) CONH

2in-plane bend

40 /14

c(NCO) NCO out-of-plane bend

41 q4 15

q(NH2) NH

2torsion

42 q1 2

q(CO2

É) CO2

É torsion

43 q2 8

q(NH3

½) NH3

½ torsion

44 q2 3

q(CC) CC torsion

45 q3 4

q(CC) CC torsion

a Arbitrary numbering.b The atomic numbering is defined in Fig. 1. is the stretching vibration of the bond between atoms i and j ; isr

i jb

i j kthe in-plane bending vibration of the angle between atoms i , j and k ; is the out-of-plane bending vibration of the/

iatom i ; is the torsion vibration respect to the bond between atoms i and j , which were defined as described in Refsq

i j35 and 36.

the CÈH and NÈH stretching bands not being observedin the spectrum of As will be conÐrmedasparagine-h5 .later by the normal mode calculation, we assumed thatthese three l(CÈH) bands do not shift appreciably upondeuteration, thus appearing at the same wavenumbersfor the natural derivative. Our reported studies on solidasparagine21 and other deuterated amino acids17h19support this assumption.

Two bands were measured in the Raman spectrum ofasparagine in solution between 1700 and 1600D2Ocm~1. They were assigned to the amide I and la(CO2~)vibrations. The wavenumber proposed for the last

vibration, 1628 cm~1, is supported by those reportedfor amino acids in solutions, such as glycine,32D2O1623 cm~1 and aspartic acid,33 1625 cm~1. The 1700È1500 cm~1 region of the Raman spectrum of

is Ðlled by the bending vibration ofasparagine-h5 H2Omolecules, but we measured bands at 1611 and 1538cm~1 in the infrared spectrum. The smaller wavenum-ber bands was assigned to a bending vibra-da(NH3`)tion because it is not observed for Theasparagine-d5 .band at 1611 cm~1 should correspond to the la(CO2~)mode, but the wavenumber measured for the deuteratedderivative, 1628 cm~1, suggests di†erences between the



Figure 2. Raman spectrum of asparagine in solution between 1800 and 400 cmÉ1.H2O

normal coordinates. The bands at 1450 and 1404 cm~1for have negligible shifts upon deuter-asparagine-h5ation, so that they can be tentatively assigned to d(CH2)and vibrations, respectively. Between them, ala(CO2~)band was measured at 1421 cm~1 for andasparagine-h5at 1418 cm~1 for We assigned it to theasparagine-d5 .amide III vibration, that largely has a charac-l(CÈNH2)ter.34 This vibration had been assigned at 1414 cm~1 inthe Raman spectrum of solid and at 1406asparagine-h5cm~1 for solid both conÐrmed byasparagine-d5 ,21normal coordinate analysis.22

In the spectral region between 1400 and 1200 cm~1for several methylene and methyneasparagine-h5bending modes usually appear, which were assigned onthe basis of our previous work on solid asparagine. Oneof these, d(CH), downshifts by 11 cm~1 upon deuter-ation, thus indicating some coupling with other coordi-nates that are more sensitive to this isotopic labelling.This region is almost completely Ðlled by the d(OD)vibration of the solvent molecules, which hides the NÈDbending vibrations. Only a band at 1160 cm~1 isassigned as the amide II mode. The rest ofd(ND2),bands measured in the Raman spectra were tentativelyassigned on the basis of our previous work on solidasparagine. All of them exhibit moderate shifts upon

deuteration as a consequence of extensive coordinatemixing that usually appears at wavenumbers below1000 cm~1 for molecules with such a size.19,20,22

The previous discussion was taken into account toreÐne the SCRF/6È31 ] G* force Ðeld of asparagine.Pure ab initio force constants were used as the startingpoint of an iterative process in which Ðve di†erent scalefactors were used. These factors arranged the 45 internalcoordinates deÐned for asparagine into Ðve groupsaccording to their characters, as is described in Table 3.The scale factor averaged value, 0.818, is close to thatusually accepted (0.81) to compensate for the systematicdeviations of such a calculation.35h37 Tables 4 and 5summarize the results obtained for the two moleculesstudied.

The theoretical wavenumber descriptions listed inTables 4 and 5 supported the prior assignments in mostcases for both and The potentialasparagine-h5 -d5 .energy distribution shows critical di†erences for the twomolecules, especially concerning wavenumbers lowerthan 1800 cm~1. It is interesting to compare some ofthe calculated wavenumbers with those reported forsolid asparagine monohydrate.38 For instance, the NH2stretching vibrations were measured at 3384 and 3112cm~1, in good agreement with the isotopic substitutions

Table 3. Scaling factors applied to the 6–31 + G*/SCRF force Ðeld ofasparagine

Coordinatesa Description Factor

1–3, 9, 18, 32–34, 44, 45 Skeletal vibrations 0.77

4, 5, 8, 17, 19–21, 30, 31 Methylene vibrations 0.81

6, 10 11, 22–24, 42 Carboxylate vibrations and CxO 0.80

7, 15, 16, 35–41 Amide vibrations (except CxO) 0.83

12–14, 25–29, 43 Ammonium vibrations 0.88

a The numbering corresponds to the coordinates defined in Table 2.



Figure 3. Raman spectrum of asparagine in solution: (a) 3400–2800 cmÉ1 region; (b) 1800–400 cmÉ1 region.D2O

and previous data on other amino acids. These wave-numbers indicate the presence in the crystal of selectiveintermolecular hydrogen bonds involving the NH2group, which are not easily reproducible by means ofsolvent e†ect calculations. The use of solvent environ-ments to predict solid-state wavenumbers does not takeinto account the speciÐc interactions present in the crys-tals. As an example, our ab initio calculation gives forthe same vibrations 3558 and 3449 cm~1. In this case,to Ðt the SCRF wavenumbers to the experimental mea-surements it would be necessary to use two di†erentscale factors for the stretching vibrations, andNH2many others for the whole molecule.

For natural asparagine, extensive coupling isobserved among l(CxO) andla(CO2~), da(NH3`),

vibrations involving wavenumbers, alld(NH2) l10Èl13

of them between 1700 and 1620 cm~1, and in the sameway for d(CH) and that involve wave-ls(CO2), l17Èl19numbers between 1390 and 1310 cm~1. The isotopicshifts of and vibrations clarify the Ðrstd(NH3`) d(NH2)region, so that the coupling does not occur for the deu-terated derivative and the descriptions change signiÐ-cantly. This e†ect arises also in some others regions,making the wavenumber comparison difficult. Never-theless, the Ðtting between theoretical and experimentalspectra can be considered satisfactory for the two mol-ecules, the di†erences being smaller than ^10 cm~1 formost of the wavenumbers.

To extend the evaluation criteria of this quantum-mechanical force Ðeld, Fig. 4 shows the calculated infra-red spectrum together with the experimental spectrumbetween 1700 and 1100 cm~1. Strong infrared absorp-



Table 4. Scaleda 6–31 + G*/SCRF and experimental wavenumbers (m8 ) for asparagine-h5

Scaled Exp.

Vibrationb l8 /cmÉ1 l8 /cmÉ1 Difference P.E.D. (greater than 10%)c

l1

3558 100la(NH

2)

l2

3503 99la(NH

3½)

l3

3457 63la(NH

3½) ½23l

a(NH

2) ½11l

s(NH

3½)

l4

3449 76ls(NH

2) ½18l

a(NH

3½)

l5

3363 83ls(NH

3½)

l6

2966 2965 1 87la(CH

2) ½13l

s(CH

2)

l7

2939 2945 É6 96l(CH)

l8

2911 2907 4 84ls(CH

2) ½13l

a(CH

2)

l9

1742 90da(NH

3½)

l10

1693 38l(CxO) ½33da(NH

3½) ½12l(C–NH

2) ½11d(NH

2)

l11

1664 38da(NH

3½) ½24l(CO

2É) ½22d(NH

2) ½12l(CxO)

l12

1642 42l(CO2

É) ½41d(NH2) ½12l(CxO)

l13

1629 1611 18 36l(CO2

É) ½23da(NH

3½) ½20d(NH

2) ½20l(CxO)

l14

1529 1538 É9 92ds(NH

3½)

l15

1444 1450 É6 72d(CH2) ½12l(CC) ½11u(CH

2) ½10l(C–NH

2)

l16

1431 1421 10 37d(CH2) ½26u(CH

2) ½15l(C–NH

2) ½12l(CC)

l17

1388 1404 É16 33d(CH) ½24l(CO2

É) ½15l(CC) ½12d(CO2

É)

l18

1361 1356 5 81d(CH) ½13l(CC) ½12l(CO2

É)

l19

1311 1329 É18 32d(CH) ½29n(CO2

É) ½10l(CC) ½10d(CO2

É)

l20

1305 1314 É9 37u(CH2) ½27l(C–NH

2) ½10l(CO

2É)

l21

1229 1227 2 68t(CH2) ½18d(CH) ½11t(CH

2)

l22

1138 1151 É13 43r(NH3

½) ½20d(CH) ½11t(CH2)

l23

1111 1120 É9 39r(NH2) ½17r(NH

3½) ½11l(CxO)

l24

1088 35r(NH3

½) ½18r(NH2) ½10d(CH)

l25

1016 983 33 22l(CC) ½18r(CH2) ½14l(C–NH

3½) ½10r(NH

3½)

l26

955 926 29 34l(CC) ½25r(NH3

½) ½23r(CH2)

l27

868 864 4 35l(C–NH3

½) ½22d(skel) ½16l(CC)

l28

849 827 22 35l(CC) ½19d(skel) ½18d(CO2

É) ½13c(OCOÉ)

l29

774 800 É26 44l(CC) ½27c(OCOÉ)

l30

771 765 6 39d(CO2

É) ½25l(C–NH3

½) ½14l(CC) ½10d(skel)

l31

674 27d(CONH2) ½14d(skel) ½11c(NCO) ½10c(OCOÉ)

l32

630 605 25 35q(NH2) ½17c(NCO) ½13d(CONH

2)

l33

556 21d(CONH2) ½18c(NH

2) ½17l(CC) ½10d(CO

2É)

l34

530 63c(NH2)

l35

512 520 É8 46d(CO2

É) ½33q(NH2) ½20c(NCO) ½17c(NH

2)

l36

473 26d(CO2

É) ½24c(NCO) ½23q(NH2) ½13c(NH

2) ½11q(skel)

l37

448 50d(skel) ½20d(CONH2) ½11c(OCOÉ) ½11l(CC)

l38

346 43d(CONH2) ½35d(skel)

l39

314 64d(skel) ½11d(CO2

É)

l40

257 101q(skel) ½22d(skel) ½21r(NH3

½)

l41

232 53d(skel) ½16d(CO2

É) ½11q(skel) ½10d(CONH2)

l42

203 53d(skel) ½35q(skel) ½12q(NH3

½)

l43

91 105q(CO2

É) ½22d(skel) ½11d(CO2

É)

l44

84 93q(skel) ½33d(skel)

l45

48 100q(skel) ½10c(NH2)

a Scaling factors from Table 3.b Arbitrary numbering.c Coordinates were defined according to Table 2. Contributions from coordinates with the same characterwere added to clarify the normal mode descriptions.

tions from water molecules do not allow the completespectrum to be observed. We used pure ab initio wave-numbers uniformly scaled by 0.81 (applied to the forceconstants) so as not to modify the infrared intensities.Figure 4 shows a reliable prediction of the experimentalspectrum, at least at the semi-quantitative level, thussupporting the results provided in the present work.

CONCLUSIONS

The Raman spectra of the amino acid asparagine andits deuterated derivative were recorded. AND2`, ND2general assignment of the measured bands was pro-posed on the basis of the isotopic shifts upon deuter-



Table 5. Scaleda 6–31 + G*/SCRF and experimental wavenumbers (m8 ) for asparagine-d5

Scaled Exp.

Vibrationb l8 /cmÉ1 l8 /cmÉ1 Difference P.E.D. (greater than 10%)c

l1

2965 2965 0 87la(CH

2) ½13l

s(CH

2)

l2

2939 2941 É2 96l(CH)

l3

2911 2907 4 83ls(CH

2) ½17l

s(CH

2)

l4

2637 99la(ND

2)

l5

2587 99la(ND

3½)

l6

2544 96la(ND

3½)

l7

2497 98ls(ND

2)

l8

2417 96ls(ND

3½)

l9

1662 1645 17 80l(CxO) ½14l(C–ND2)

l10

1638 1628 10 99l(CO2

É)

l11

1450 1451 É1 30d(CH2) ½27l(C–ND

2) ½20n(CC) ½18u(CH

2)

l12

1436 1418 18 81d(CH2)

l13

1382 1406 É24 36l(CO2

É) ½18l(CC) ½17d(CH) ½10d(CO2

É)

l14

1347 1345 2 71d(CH) ½10o(CH2)

l15

1320 1328 8 26u(CH2) ½25l(CO

2) ½12l(CxO) ½11d

a(ND

2) ½11l(CC)

l16

1299 53d(CH) ½15l(CO2) ½15l(CC) ½10u(CH

2)

l17

1252 95da(ND

3½)

l18

1226 67t(CH) ½16d(CH)

l19

1195 85da(ND

3½)

l20

1180 80ds(ND

3½) ½15l(C–ND

3½) ½11d

a(ND

3½)

l21

1161 1160 1 61d(ND2) ½11d(COND

2)

l22

1061 1081 É20 35l(CC) ½14d(CH) ½12r(ND3

½)

l23

1016 1043 É27 34r(CH2) ½18l(C–ND

3½) ½12d(skel) ½12c(NCO)

l24

976 1003 É27 39d(skel) ½16r(ND3

½) ½11c(OCOÉ) ½10r(ND2)

l25

922 919 3 28r(ND2) ½12r(ND

3½) ½10d(skel)

l26

855 875 É20 23l(CC) ½21r(CH2) ½15l(C–ND

3½) ½14d(CO

2É)

l27

819 825 É6 44r(ND3

½) ½40l(CC) ½86d(CO2)

l28

770 786 É16 27r(ND3

½) ½22d(CO2

É) ½21l(CC) ½18l(C–ND3

½)

l29

753 748 5 18r(ND3

½) ½17d(CO2

É) ½15c(OCOÉ) ½12l(C–ND3

½)

l30

733 695 38 34c(OCOÉ) ½33l(CC) ½11r(ND2)

l31

634 655 É21 22d(COND2) ½18c(NCO)

l32

581 571 10 41c(NCO) ½17r(CH2)

l33

519 25d(COND2) ½25d(skel) ½14d(CO

2) ½11r(ND

2)

l34

478 44d(CO2)

l35

425 52d(skel) ½23d(COND2) ½12l(CC)

l36

410 78c(ND2) ½12q(ND

2)

l37

378 79q(ND2) ½33c(ND

2) ½13c(NCO)

l38

320 40d(COND2) ½33d(skel)

l39

286 77d(skel) ½11t(skel)

l40

223 43d(skel) ½18d(CO2

É) ½18q(skel) ½10d(COND2)

l41

205 45d(skel) ½22q(skel)

l42

175 97q(ND3

½) ½17d(skel) ½12r(ND3

½)

l43

90 103q(CO2

É) ½11d(CO2)

l44

81 95q(skel) ½33d(skel)

l45

46 100q(skel)

a Scaling factors from Table 3.b Arbitrary numbering.c Coordinates were defined according to Table 2. Contributions from coordinates with the same characterwere added to clarify the normal mode descriptions.



Figure 4. Comparison between SCRF/6–31 ½G* (top) andexperimental (bottom) infrared spectra of asparagine in the 1700–1100 cmÉ1 region.

ation and previously reported work on solid asparagineand related molecules. Quadratic force constants forthis molecule were computed at the RHF/6È31G* level.The polar environment was simulated using the SCRFtheory. Previous geometry optimization provided a setof structural data that were in good agreement with theexperimental x-ray structures of monohydrated aspara-gine. Ab initio force constants were scaled by only Ðvescaling factors, all of them around 0.81. The observedwavenumbers were reliably reproduced and the normalmode descriptions supported the proposed assignmentsin the most cases. Infrared absorption intensities werealso successfully compared with experiments. Theresults conÐrm the ability of this methodology to inter-pret the vibrational spectrum of such a molecule in apolar environment. In addition, more dramatic correc-tions are needed to reproduce wavenumbers from thesolid state, even though a good correlation betweentheoretical and observed structures exists.

Acknowledgements

J.C. is grateful to the Junta de Andaluci� a (Group No. 6072) for Ðnan-cial support. We are also grateful to the CICA Computer Center ofSevilla (Spain) for computing facilities.

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Experimental and theoretical vibrational study of methylene bis(thiocyanate), CH2(SCN)2. A...

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