Thermochemical parameters of caffeine, theophylline, and xanthine

J. Chem. Thermodynamics 42 (2010) 437–440

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Thermochemical parameters of caffeine, theophylline, and xanthine Ngo Tuan Cuong a, Truong Ba Tai a, Vu Thi Thu Ha b, Minh Tho Nguyen a,* a b

Department of Chemistry, and Mathematical Modeling and Computational Science Center (LMCC), Katholieke Universiteit Leuven, B-3001Leuven, Belgium Institute of Chemistry, Vietnam Academy of Science and Technology, Hanoi, Viet Nam

a r t i c l e

i n f o

Article history: Received 9 July 2009 Received in revised form 15 October 2009 Accepted 23 October 2009 Available online 29 October 2009 Keywords: Caffeine Theophylline Xanthine Uracils Imidazoles Heats of formation Ionization energies Quantum chemical calculations

a b s t r a c t Thermochemical parameters of caffeine 1, theophylline 2, xanthine 3, uracil, and imidazole derivatives are determined by quantum chemical calculations. Using the composite G3B3 method, the standard heat 1 of formation of caffeine in the gaseous phase amounts to Df Hg ð1Þ ¼ 243  8 kJ  mol , which lends a 1 support for the recent experimental value of 237.0 ± 2.5 kcal  mol . We also obtain Df Hg ð2Þ ¼ 1 1 232  8 kJ  mol and Df Hg ð3Þ ¼ 209  8 kJ  mol . The adiabatic ionization energies are IEa(1) = 7.9 eV, IEa(2) = 8.1 eV, and IEa(3) = 8.5 eV using B3LYP calculations. The enhanced ability of caffeine to eject electron, as compared to the parent compounds and cyclic components, is of interest with regard to its potential use as a corrosion inhibitor. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Caffeine 1 (scheme 1) is a biologically active substance and exerts many pharmacological effects on the human body. It is firmly established in the popular culture as a stimulant for the central nervous system, and therefore it is omnipresent and consumed every day by billions of people from a variety of foods and drinks. In low concentrations, caffeine inhibits the adenosine moiety of the cyclic adenosine monophosphate (c-AMP) to bind to its receptor, but at high concentrations, it becomes an inhibitor of phosphodiesterase [1,2]. Caffeine can play the role of an effective antioxidant, in particular in the scavenging of the hydroxyl radical [3,4]. 1 was proved to be a good indicator in analytical methods to detect microbes in surface waters [5], or potentially useful starting reagent in the synthesis of N-heterocyclic carbenes [6]. More interestingly, 1 has been proved to be a promising inhibitor against the corrosion process of many metal and alloys [7–9]. The use of natural organic compounds such as caffeine and green tea to protect metals from corrosion is particularly attractive, because they are both economic and environment-friendly. The effect likely arises from their ability to induce strong adsorption onto the metal surface, which prevents the latter from undergoing oxidation reactions.

* Corresponding author. Tel.: +32 16 32 7361; fax: +32 16 32 7992. E-mail address: [email protected] (M.T. Nguyen). 0021-9614/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2009.10.006

Caffeine can be extracted not only from coffee and cocoa beans, but also from green tea leaves, etc. In view of its popularity, chemical properties and reactions of caffeine have much been investigated during the last two centuries. Its structural and spectroscopic parameters were well determined. Its molecular geometry was determined by crystal structure analysis [10,11] and its various spectra in different forms well recorded and assigned (infrared: [12,13], UV–visible: [14–16], He I photoelectron: [17], and photoionization: [18]). Although the thermochemical parameters of caffeine have been the subject of a number of studies [19–32], some fundamental values remain the subject of recent debate. The experimental values previously determined for the sublimation enthalpy of 1 in the crystal phases vary from (104 to 117) kJ  mol1. For the standard enthalpy of formation of caffeine in the crystal phase, Pinto and Diogo [30] derived from combustion experiments the values of (345.1 ± 2.3 and 340.6 ± 2.3) kJ  mol1, the both a and b phases, respectively. Subsequently, Dong et al. [31] obtained by using bomb combustion calorimetry in oxygen, a value of 322.2 ± 4.8 kJ  mol1 for this quantity in the a-phase, which thus differs significantly from the earlier results [30]. More recently, Emel’yamenko and Verevkin [32] redetermined the heat of sublimation of 1 using the transpiration method, and their results were more consistent with those of Pinto and Diogo [30]. Using the set of enthalpies of formation and sublimation in the crystal phase reported in references [30,32], the standard enthalpy of formation in the gaseous phase Df Hg ð1Þ can be evaluated to be

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SCHEME 1.

237.0 ± 2.5 kJ  mol1 from data for a-phase, and 236.4 ± 4.3 kJ  mol1 from data for b-phase. Quantum chemical calculations at the composite G2 level provided a value of Df Hg ð1Þ ¼ 1 235:5 kJ  mol [32], which is closer to the latter experimental evaluation. Caffeine 1 is formally a trimethylated derivative of xanthine 3 (scheme 1), which is a parent of the class of oxopurines, and a precursor of the nucleobase guanine. 1 is also a higher methylated homologue of theophylline 2. As such, either 1, 2 or 3, arises from a fusion of the six-membered uracil 4 and 5 and the five-membered imidazole 6 and 7, whose properties are well known. The coupling of two typical chromophores is of interest as for the relationship between color and chemical constitution. In view of the current debate on the thermochemical parameters of caffeine, we set out to determine its gas phase heat of formation of using advanced quantum chemical methods. In addition, the vertical and adiabatic ionization energies are also determined. For the sake of comparison, the parameters of theophylline 2 and xanthine 3, and the smaller components 4–7, shown in scheme 1, are evaluated. 2. Computational methods All quantum chemical calculations are carried out using the Gaussian 03 package of programs [33]. Two sets of calculations

are considered. In the first set, geometry and harmonic vibrational frequencies are initially determined using the density functional theory with the hybrid B3LYP functional, in conjunction with the polarized plus diffuse 6-311++G(d) basis set. Geometries of the relevant equilibrium structures are subsequently reoptimized using the B3LYP method but with the larger aug-cc-pVTZ basis set. The later calculations allow both vertical and adiabatic ionization energies to be directly evaluated. In the second set of calculations, the composite method G3B3 [34] is used to calculate the energies of atomization RD0. Together with the experimental heats of formation of the elements, the enthalpies of formation of the molecules and cations in the gaseous phase, Df Hg , are determined at both temperatures of (0 and 298) K. The unrestricted formalism (UHF, UB3LYP) has been used for open-shell systems. 3. Results and discussion Table 1 lists the calculated total atomization energies (RD0), the gaseous phase enthalpies of formation (DfH) at both (0 and 298) K for the seven species considered in both lowest-lying neutral and cationic states. The vertical ionization energies (VIE) are determined using (U)B3LYP/aug-cc-pVTZ calculations. The adiabatic ionization energies (AIE) are evaluated using (U)B3LYP/aug-cc-pVTZ + ZPE calculations and G3B3 heats of

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TABLE 1 Total atomization energy (RD0), heats of formation (DfHg at 0 K and 298 K), vertical (VIE), and adiabatic (AIE) ionization energies of caffeine, theophylline, and xanthine and derivatives. Species Caffeine 1 Caffeine cation 1+ Theophylline 2 Theophylline cation 2+ Xanthine 3 Xanthine cation 3+ Uracil 4 +

Uracil cation 4 Methyluracil 5 Methyluracil cation 5+ Imidazole 6 Imidazole cation 6+ N-methylmidazole 7 N-methylmidazole Cation 7+ a b c d e f g

Electronic state

RD0/

DfHg 0 K, G3B3/(kJ  mol1)

DfHg 298 K G3B3/(kJ  mol1)

DfHg exptl 298 K/(kJ  mol1)

VIEcalc B3LYP/eV

AIEcalc G3B3/eV

AIEcalc B3LYP/eV

VIEexptl/eV

(kJ  mol1)

1

10437.8 9666.0 9289.2 8498.4 6989.9 6154.7 5427.2

202.8 568.9 197.5 593.3 184.6 650.6 274.8

243.4 526.2 231.7 559.2 208.8 626.3 294.0

237.0 ± 2.5

8.06

8.00

7.87

8.25a

8.27

8.20

8.07

8.74

8.66

8.51

8.84a

303.1 ± 2.3b,c

9.52

9.35

9.21

9.68; 9.60 9.45; 9.50 9.59c

4524.7 7723.7 6876.1 3789.3 2936.8 4935.3 4110.2

627.7 284.8 562.8 150.1 1002.5 147.2 972.4

608.8 314.2 533.7 134.0 986.6 126.0 949.5

313.6 ± 1.5c,d

8.84

8.79

8.62

132.9e 133.0 993.8f 137.8 ± 4g

8.99

8.84

8.71

8.96c

8.78

8.55

8.43

8.66c

A0 A00 1 0 A 2 00 A 1 0 A 2 00 A 1 0 A 2

2

00

A A0 2 00 A 1 0 A 2 0 A 1 0 A 2 00 A 1

Reference [18]. Reference [42]. Reference [41]. Reference [45]. Reference [35,36]. Reference [38]. Reference [42].

formation at 0 K. Table 1 also includes the available experimental values for comparison. For imidazole 6, the simplest species considered, the G3B3 value of 134.0 kJ  mol1 for its heat of formation at 298 K compares indeed quite well with the experimental results of 132.9 ± 0.6 kJ  mol1 [35] and 133.0 ± 1.7 kJ  mol1 [36]. A recent study using coupled-cluster CCSD(T) theory with extrapolated complete basis set (CBS) obtained a value of 132.2 kJ  mol1 [37]. On the contrary, the G3B3 value for the radical cation 6+ deviates by 6 kJ  mol1 with respect to the experimental results [38]. This is however in line with the performance of various theoretical methods for both VIE and AIE of imidazole [39]. In this case, the B3LYP value of 8.71 eV for AIE(6) is closer to the photoelectron result of 8.67 eV [39,40] than the G3B3 value of 8.84 eV. The B3LYP value for VIE(6) compares also quite good with experiment [41]. It appears that the IEs determined from B3LYP calculations are systematically smaller by 0.15 eV than those from G3B3 heats of formation. This suggests that the heats for formation of the radical cations are underestimated by a similar amount. This is likely due to the spin contamination in the UHF wavefunctions that slows down the convergence of the MPn and QCISD expansions in the G3 approach. For N-methylimidazole 7, the G3B3 heat of formation is 126.0 kJ  mol1 at 298 K, ca. 12 kJ  mol1 less endothermic than the experimental value of 137.8 ± 4 kJ  mol1 determined by Mo and co-workers [42]. This value is closer to the values obtained by using the total atomization energies of (128.5 and 125.1) kJ  mol1 at the G2(MP2) and B3LYP/6-311+G(3df,2p)//B3LYP/631G(d) levels, respectively, than those obtained using quasiisodesmic reactions, namely 132.6 (G2MP2) and 138.5 kJ  mol1, B3LYP/6-311+G(3df,2p) [42]. The agreement of the calculated VIE is also in the order of 0.1 eV. It appears that an N-methylation of the five-membered ring reduces its ionization energy by 0.2 eV. The G3B3 value of 294.0 kJ  mol1 for DfH (298 K) of uracil 4 is underestimated by 9 kJ  mol1 with respect to the NIST value [41,43] (table 1) but closer to an earlier theoretical estimate of 295.4 kJ  mol1 [44]. On the contrary, the values for AIE(4) of (9.2 to 9.3) eV compare well with the experimental result of

9.35 eV [45,46]. In addition, the calculated heat of formation of methyluracil 5 differs by <1.0 kJ  mol1 from experiment. All these calibrations suggest that the current experimental heat of formation of uracil 4 of 303.1 kJ  mol1 is overestimated by a few kJ  mol1. The average of both G3B3 and NIST values could be considered as a better estimate for this quantity, namely DfH(uracil) = 298 ± 4 kJ  mol1 at 298 K. For both caffeine 1 and xanthine 3, the calculated VIE’s are consistently smaller by (0.1 to 0.2) eV with respect to the photoionization results [18]. Overall, comparisons with the rather limited experimental data point out that the accuracy of the G3B3 heats of formation of the neutral compounds lies within that expected for this composite method, namely at most ±8 kJ  mol1 [34]. In this context, the same error bar can be applied to the calculated values for the neutral state of caffeine 1, theophylline 2, and xanthine 3. While the G3B3 heats of formation for the cations are relatively less accurate, the IEs derived from B3LYP calculations appear to be more reliable. Thus, the upper bound of the calculated G3B3 value for DfH(caffeine) = 243.4 ± 8 kJ  mol1 at 298 K is actually overlapping with the most recent experimental estimates of 237.0 ± 2.5 (a-phase) kJ  mol1 and 236.4 ± 4.3 (b-phase) kJ  mol1 [32]. Because the latter values were derived indirectly from measurements on solid state materials, whereas calculations are designed for gasphase systems, such agreement is quite reasonable. A small difference also emerges with respect to the previous G2 value for caffeine [32]. As for an estimate, we consider the average of both G3B3 and experimental results, namely 240 ± 8 kJ  mol1, as our prediction for the standard heat of formation of caffeine 1 in the gas phase. The calculated results for theophylline and xanthine are given in table 1 with the same error bar. The difference between the enthalpies of formation of xanthine in condensed phase and in the gas phase, i.e. the enthalpy of sublimation of xanthine, is 170.8 kJ  mol1. This value seems to be large with respect to the enthalpies of sublimation of caffeine (97.1 kJ  mol1), theophylline (143 kJ  mol1 and 126 kJ  mol1), uracil (84 kJ  mol1), 1,3-dimethyluracil (97 kJ  mol1), imidazole (83 kJ  mol1), and N-methylimidazole (64.7 kJ  mol1). A likely reason for such a difference is the presence of several inter-

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molecular H-bonds between xanthine molecules in the crystal structure. The IE tends to decrease not only upon fusion of both uracil and imidazole components, which obviously facilitates electron delocalization, but also following methylation. Accumulation of both effects reduces substantially the IE by up to 1.35 eV in going from uracil 4 to caffeine 1. Thus, caffeine turns out to possess the lowest IE of 7.9 eV. Its enhanced ability to lose one electron is an interesting property with regard to its potential use as an inhibitor against metal corrosion. Acknowledgements The authors are indebted to the KULeuven Research Council (GOA, IUAP and IDO Programs) for continuing support. NTC thanks the Government of Vietnam for a partial doctoral scholarship (MOET – Program 322). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

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JCT 09-243