Ion [C5H5O]C formation in the electron-impact mass spectra of 4-substituted

N-(2-furylmethyl)anilines. Relative abundance prediction ability of the DFT

calculations

Eduardo Solano *, Elena Stashenko, Jairo Martınez, Uriel Mora, Vladimir Kouznetsov

Centro de Investigacion en Biomoleculas, CIBIMOL. Escuela de Quımica, Universidad Industrial de Santander, A. A. 678, Bucaramanga, Colombia

Received 10 March 2006; accepted 24 April 2006

Available online 6 May 2006

Abstract

Six 4-substituted N-(2-furylmethyl)anilines have been studied by electron ionization (EI) mass spectrometry. The heterolytic dissociation of the

C–N bond of the molecular ions was proposed as the route to the main fragment ion in the mass spectra, [C5H5O]C (m/zZ81). Reaction energies

for this process were calculated at UB3LYP/6-31G(d,p) and UHF/6-31G(d,p) levels of theory. DFT calculations produced reaction energy values

that showed very good correlation with the logarithms of the peak intensity ratios for the [C5H5O]C and [M]C% ions. Linear regression yielded an

equation, which could be used to calculate the intensity ratio [C5H5O]C/[M]C%, with an error !10%, from both the analogous ratio in a known

mass spectrum and theoretical reaction energies.

q 2006 Elsevier B.V. All rights reserved.

Keywords: 4-Substituted N-(2-furylmethyl)anilines; Ion [C5H5O]C; Mass spectrometry; UB3LYP/6-31G(d,p); and UHF/6-31G(d,p); critical energies

1. Introduction

The N-(2-furylmethyl)anilines are precursors in the

synthesis of complex bioactive molecules [1,2]. The

[C5H5O]C ion can be originated either via direct dissociation

or via isomerization and fragmentation of molecular ions [3].

However, we assumed the former pathway (Fig. 1), taking into

account the analogous case of hydrogen loss from the methyl-

benzene ion, in which dissociation predominates when

molecular ions have high energies and rearrangement predo-

minates when they have low energies [4].

The peak intensity ratios written in Eq. (1), were used here

as a measurement of [C5H5O]C relative abundance. The

numerator is calculated from each of the six mass spectra

and the zero superscripts refer to a reference mass spectrum,

that is, the unsubstituted molecule.

Z

Z0

ðC5H5OCÞZ

½C5H5O�C=½M�C$

½C5H5O�C0 =½M0�

C$ (1)

0166-1280/$ - see front matter q 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.theochem.2006.04.034

* Corresponding author. Tel.: C57 7 6456737; fax: C57 7 6358210.

E-mail address: elena@tucan.uis.edu.co (E. Solano).

In this work, we have explored the possibility of calculating

Z, by means of DFT and HF methods.

2. Experimental

The compounds were synthesized via Schiff base method

from furfural and the respective substituted aniline. The

proposed structures for were based on their analytical and

spectroscopic data (FTIR, 1H NMR, 13C NMR, MS) [1]. The

mass spectra were obtained using an Agilent Technologies

6890 Plus gas chromatograph coupled to Agilent Technologies

5173N mass selective detector. A DB-1 MS (poly(dimethil-

siloxane)) fused silica capillary column, 60 m!0.25 mm!0.25 mm, was employed.

All calculations were carried out using the GAUSSIAN 03

computational package [5]. The geometric parameters for all

molecular and fragment ions and radicals involved in the

fragmentation process were fully optimized at the UHF/

6-31G(d,p) and UB3LYP/6-31G(d,p) levels of theory. Each

stationary structure was characterized as a minimum by

frequency calculations.

3. Results and discussion

The EI mass spectra of 1–6 are listed in Table 1. The base

peaks in all of them, except that of 6, correspond to the

Journal of Molecular Structure: THEOCHEM 769 (2006) 83–85

www.elsevier.com/locate/theochem

Fig. 1. Ion [C5H5O]C formation from molecular ions of 4-substituted N-

(2-furylmethyl)anilines dissociation.

Table 2

Electronic energies, evaluated at the UHF/6-31G(d,p) and UB3LYP/6-

31G(d,p) levels of theory, and vibrational energies (ZPE) in Hartree for all

fragment, molecular ions and radicals involved in the main fragmentation

Species UHF/6-31G(d,p) UB3LYP/6-31G(d,p)

E(UHF) ZPE E(UBCHF-LYP)

ZPE

Molecular ions

RaBr K3121.344653 0.198298 K3126.606174 0.187107

RaCl K1010.937214 0.198829 K1015.095838 0.187502

RaF K650.888715 0.200442 K654.734086 0.188958

RaH K552.046691 0.208773 K555.504258 0.197012

RaCH3 K591.091040 0.238169 K594.831655 0.224288

RaOCH3 K665.934569 0.244919 K670.046176 0.229991

Radicals

RaBr K2854.456641 0.098463 K2858.068492 0.093960

RaCl K744.049625 0.098955 K746.559685 0.094360

RaF K383.999683 0.100434 K386.197158 0.095761

RaH K285.151458 0.108818 K286.965020 0.103877

RaCH3 K324.191937 0.138002 K326.286778 0.131255

RaOCH3 K399.031770 0.143522 K401.493073 0.136750

Ion C5H5OC K266.860806 0.095412 K268.484085 0.089263

E. Solano et al. / Journal of Molecular Structure: THEOCHEM 769 (2006) 83–8584

[C5H5O]C ion. Assuming the structure of furfuryl for this ion,

the critical fragmentation energies at 0 K at the UHF/

6-31G(d,p) and UB3LYP/6-31G(d,p) levels of theory were

calculated. The energies for all fragment ion, molecular ions

and radicals are listed in Table 2. Table 3 shows the critical

energies, E0.

The experimental values of Z listed in Table 4 are strongly

substituent-dependent. Fig. 2 shows plots of ln Z/Z0

against the differences of critical energies, DE0ZE0KE00,

where E00 corresponds to RaH. The correlation was poor

when E0 were calculated with the HF method

(slopeZK0.0173; r2Z0.79), while DFT E0 exhibited a

good correlation (slopeZK0.0174; r2Z0.95). Linear

regressions yielded expressions such as Eq. (2), where m is

the slope and b is the intercept.

lnZ

Z0

Zm!DE0 Cb (2)

Table 1

The EI 69.9 eV mass spectra, showing mostly peaks of R2% relative

abundance (RA), of compounds 1–6

Compound m/z (% RA)

1 253(M, 36) 252(30) 251(M, 37) 250(27) 225(3) 223(3)

184(2) 172(2) 171(3) 170(2) 157(3) 155(3) 145(2) 143(4)

117(2) 115(3) 104(2) 91(5) 82(6) 81(100) 77(2) 76(3) 75(3)

64(2) 63(4) 53(14) 52(2) 51(3) 50(2)

2 209(M, 16) 208(15) 207(M, 47) 206(29) 179(3) 143(2)

140(3) 138(2) 127(2) 126(2) 125(2) 115(2) 113(2) 112(1)

111(5) 99(4) 82(6) 81(100) 77(2) 75(5) 73(2) 63(3) 53(14)

52(2) 51(3) 50(2)

3 191(M, 65) 190(40) 163(5) 162(4) 161(2) 135(2) 133(3)

124(3) 122(5) 111(2) 110(2) 109(3) 96(3) 95(9) 83(7) 82(7)

81(100) 75(5) 57(3) 53(17) 52(2) 51(3)

4 173(M, 81) 172(52) 156(2) 145(9) 144(9) 143(3) 130(3)

128(2) 118(2) 117(3) 115(4) 106(4) 104(4) 93(2) 92(2) 91(4)

82(6) 81(100) 78(2) 77(12) 66(2) 65(6) 63(2) 53(18) 52(2)

51(7) 50(2)

5 187(M, 88) 186(67) 170(3) 159(6) 158(7) 144(6) 143(3)

142(2) 130(2) 120(3) 118(3) 115(2) 107(2) 106(7) 105(3)

92(2) 91(9) 89(2) 82(6) 81(100) 79(4) 78(4) 77(9) 65(5)

63(2) 53(15) 52(3) 51(4)

6 203(M, 100) 202(51) 187(2) 186(3) 175(6) 174(3) 160(5)

159(2) 158(2) 136(2) 134(2) 130(3) 123(8) 122(88) 121(4)

108(5) 107(2) 95(11) 92(3) 82(4) 81(69) 80(3) 79(79) 78(2)

77(4) 67(2) 65(3) 64(3) 63(3) 53(14) 52(5) 51(3) 41(2)

Z was calculated using Eq. (3). In this equation, A is a

constant, AZZ0eb. Calculated Z are shown in Table 4. The

best values are those calculated by DFT, which present

relative errors below 10%.

Z ZA exp½mðE0KE00Þ� (3)

Table 3

Critical energies at 0 K in kJmolK1, evaluated at the UHF/6-31G(d,p) and

UB3LYP/6-31G(d,p) levels of theory, for the formation of the [C5H5O]C ion

System E0 (C5H5OC), kJmolK1

UHF/6-31G** UB3LYP/6-31G**

1 RaBr 123.814 193.997

2 RaCl 122.599 189.994

3 RaF 126.036 191.886

4 RaH 142.458 198.112

5 RaCH3 152.062 213.187

6 RaOCH3 158.537 234.238

Table 4

Theoretical and experimental values of ZZ [C5H5O]C/[M]C%

System Za[C5H5O]C/[M]C%

UHF/ 6-

31G(d,p)

UB3LYP/6-

31G(d,p)

Experimental

RaBr 1.544 1.436 1.370

RaCl 1.577 1.540 1.587

RaF 1.486 1.490 1.538

RaH 1.118 1.337 1.235

RaCH3 0.947 1.029 1.136

RaOCH3 0.846 0.714 0.690

Fig. 2. ln Z/Z0 vs DE0ZE0KE00, carrying out calculations at UHF/6-31G(d.p) (left) and UB3LYP/6-31G(d.p) (right) levels of theory.

E. Solano et al. / Journal of Molecular Structure: THEOCHEM 769 (2006) 83–85 85

4. Conclusion

In summary, we calculated the relative abundance of

[C5H5O]C ion in the mass spectrum of a given 4-substituted

N-(2-furylmethyl)aniline, from both calculated reaction energy

(UB3LYP/6-31G(d,p), in kJmolK1) and knowledge of a

reference spectrum, by means of Eq. (3). The results are in

agreement with experimental intensities.

References

[1] U. Mora, Undergraduated Thesis Universidad Industrial de Santander,

Bucaramanga, 2001.

[2] V.V. Kouznetsov, L.S. Astudillo, L.Y. Vargas, M.E. Cazar, J. Chil. Chem.

Soc. 49 (4) (2004) 319–325.

[3] R. Spilker, H.F. Grutzmacher, Org. Mass. Spectrom. 21 (1986) 459–466.

[4] P.B. Armentrout, T.J. Baer, Phys. Chem. 100 (1996) 12866–12877.

[5] M. J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R.

Cheeseman, J.A. Montgomery, Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M.

Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi,

G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara,

K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,

O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross,

C.Adamo, J. Jaramillo,R.Gomperts,R.E. Stratmann,O.Yazyev,A.J.Austin,

R.Cammi,C. Pomelli, J.W.Ochterski, P.Y.Ayala,K.Morokuma,G.A.Voth,

P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels,

M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari,

J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski,

B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin,

D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara,

M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong,

C. Gonzalez, J.A. Pople, Gaussian, Inc., Pittsburgh, PA, 2003.