Energetics and Structural Properties, in the Gas Phase, of trans

Article pubs.acs.org/JPCA

Energetics and Structural Properties, in the Gas Phase, of transHydroxycinnamic Acids Juan Z. Dávalos,* Rebeca Herrero, Antonio Chana, Andrés Guerrero, Pilar Jiménez, and José María Santiuste Instituto de Química-Física “Rocasolano”, CSIC, Serrano 119, 28006 Madrid, Spain S Supporting Information *

ABSTRACT: We have studied the energetics and structural properties of trans-cinnamic acid (CA), o-, m-, and p-coumaric acids (2-, 3-, and 4-hydroxycinnamic acids), caffeic acid (3,4-dihydroxycinnamic acid), ferulic acid (4hydroxy-3-methoxycinnamic acid), iso-ferulic acid (3-hydroxy-4-methoxycinnamic acid), and sinapic acid (3,5dimethoxy-4-hydroxycinnamic acid). The experimental values of ΔfHm°(g), determined (in kJ·mol−1) for CA (−229.8 ± 1.9), p-coumaric acid (−408.0 ± 4.4), caffeic acid (−580.0 ± 5.9), and ferulic acid (−566.4 ± 5.7), allowed us to derive ΔfHm°(g) of o-coumaric acid (−405.6 ± 4.4), m-coumaric acid (−406.4 ± 4.4), iso-ferulic acid (−565.2 ± 5.7), and sinapic acid (−698.8 ± 4.1). From these values and by use of isodesmic/homodesmotic reactions, we studied the energetic effects of π-donor substituents (-OH and -OCH3) in cinnamic acid derivatives and in the respective benzene analogues. Our results indicate that the interaction between -OCH3 and/or -OH groups in hydroxycinnamic acids takes place without significant influence of the propenoic fragment.

I. INTRODUCTION Phenolic acids, such as hydroxycinnamic acid (Scheme 1) and hydroxybenzoic acid, are a diverse group of aromatic secondary

other hand, structure−activity relationship studies have also pointed out the importance of a catechol moiety to the radicalscavenging activity, particularly p-hydroxycinnamic derivatives, whereas the contribution of the propenoic side chain remains controversial.6a,8 In spite of the important uses and applications of hydroxycinnamic acids, reliable experimental thermochemical studies are scarce. In this work, we present a systematic study on (i) the thermodynamic and structural properties, basically revising and/or determining their enthalpies of formation, and (ii) the energetic effects of the interaction among -OH, -OCH3, and propionic groups in the hydroxycinnamic acids. For these purposes, we used experimental techniques such as differential scanning calorimetry (DSC), static bomb (micro and macro) combustion calorimetry, Knudsen effusion technique, and quantum chemical calculations at the density functional theory (DFT) level. Our results are discussed in the context of isodesmic reactions,9 which are widely used in predicting molecular stability.10 In this work, the experimental ΔfHm°(g) values for several reference compounds of the considered isodesmic reactions were taken from Pedley,11 Chase12 and NIST Database13 (see Table S0 of the Supporting Information).

Scheme 1

plant metabolites. They are ubiquitously distributed, in both edible and nonedible plants, as esters or glycoside derivatives. In recent years the hydroxycinnamic acids and derivatives have attracted much attention due to their various biological,1,2 photobiological,3 and pharmaceutical activities and also to their industrial or technological applications.4 Evidence has been found about potential health benefits of the p-coumaric, caffeic, and ferulic acids.5 Many of these effects are related to their antioxidant properties, which may be due to their ability to scavenge free radicals and/or to synergistic effects with physiological antioxidants and several enzymes. In fact, these actions could prevent oxidative damage of biomolecules (proteins, membrane lipids, and nucleic acids) related to various diseases such as cancer, cardiovascular risks, or diabetes.5b The first steps of the main antioxidant mechanisms described in the literature6 are related to the donation of their phenolic hydrogen atom or to the transfer of one electron to the free radical. In this context, it is widely accepted that the antioxidant efficacy of phenolic compounds is enhanced by a relatively low phenolic O−H bond dissociation enthalpy and a relatively high ionization potential.7 On the © 2012 American Chemical Society

II. EXPERIMENTAL SECTION A. Materials and Purity Control: Differential Scanning Calorimetry Measurements. 4-Hydroxy-trans-cinnamic (pcoumaric acid, 4-OH-CA; CAS 7400-08-0) and 4-hydroxy-3methoxycinnamic (ferulic acid, 4-OH-3-OCH3-CA; CAS 113524-6) acids were purchased from Sigma−Aldrich Co., whereas Received: September 19, 2011 Revised: February 6, 2012 Published: February 8, 2012 2261

dx.doi.org/10.1021/jp2090439 | J. Phys. Chem. A 2012, 116, 2261−2267

The Journal of Physical Chemistry A

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Table 1. Experimental Determination of Standarda Molar Enthalpies of: Combustion, Formation, And Sublimation at T = 298.15 K. All values are given in kJ·mol−1 trans-CA CA, cinnamic acid 4-OH-CA, p-coumaric acid 3,4-diOH-CA, caffeic acid 4-OH-3-OCH3-CA, ferulic acid

ΔcHm°(cd), kJ·mol−1 −4146.1 ± 2.6 −3934.9 ± 2.5 −4666.0 ± 3.8

ΔfHm°(cd), kJ·mol−1 −336.9 −538.9 −750.04 −698.2

± ± ± ±

1.7 4.0 2.8 4.0

ΔcdgHm°, kJ·mol−1

b

107.1 130.9 170.0 131.8

± ± ± ±

ΔfHm°(g), kJ·mol−1

c

0.8 3.3 5.2d 4.0

−229.8 −408.0 −580.0 −566.4

± ± ± ±

1.9 4.4 5.9 5.7

a For standard enthalpies, po = 105 Pa. Enthalpies of formation are given in the condensed and gaseous states. bTaken from Pedley.11 cTaken from Monte and Hillesheim.21 dTaken from Chen and co-workers22 and computed at T = 298.15 K by use of eq 5.

experiments are represented by reactions 1−3 and are shown in Table S2 of Supporting Information.

3,4-dihydroxycinnamic acid (caffeic acid, 3,4-diOH-CA; CAS 331-39-5) was purchased from Alfa Aesar Co. All the samples were carefully dried under vacuum at 320 K and used without further purification. Purity of 4-OH-CA and 3,4-diOH-CA was checked by gas chromatography Agilent HP-5890, while 4-OH3-OCH3-CA was assessed by DSC. The mass fraction of impurities in the compounds was less than 0.005 in all cases. All samples were investigated by DSC, over the temperature range from T = 263 K to several degrees below their reported melting points, with no phase transition observed in the solid state. Molar heat capacities at constant pressure, Cp,m°, were also experimentally determined by DSC. Full details are given in the Supporting Information (section S1). B. Static Bomb Combustion Calorimetry. The combustion experiments for 4-OH-CA and 3,4-diOH-CA were carried out in an isoperibol static micro bomb calorimeter, while 4OH-3-OCH3-CA experiments were performed in a macro bomb. Detailed description of these methods is found elsewhere.14 The energy equivalent of the calorimeters, ε(calor), were determined from the combustion of benzoic acid (NIST standard reference sample 39j) with a massic energy of combustion of −26 434 ± 3 J·g−1, under certified conditions. We obtained ε(calor) of 2105.1 ± 0.3 and 14 266.3 ± 2.0 J·K−1 for micro and macro bomb calorimeters, respectively, from 10 calibration experiments. The uncertainty quoted is the standard deviation of the mean value. Additional details are given in section S2 of Supporting Information. C. Vapor Pressure Measurements. The vapor pressures as a function of temperature for 4-OH-CA and 4-OH-3-OCH3CA were measured by the mass-loss Knudsen effusion technique, according to procedures previously described.15 Full details are given in the Supporting Information (section S3).

C9H8O3 (p‐coumaric acid, cd) + 19 2 O2 (g) → 9CO2 (g) + 4H2O(l)

(1)

C9H8O4 (caffeic acid, cd) + 9O2(g) → 9CO2 (g) + 4H2O(l)

(2)

C10H10O4 (ferulic acid, cd) + 21 2 O2 (g) → 10CO2 (g) + 5H2O(l)

(3)

The standard (p° = 0.1 MPa) molar enthalpies of combustion, ΔcHm°(cd), and formation, ΔfHm°(cd), in the solid phase at temperature T = 298.15 K are shown in Table 1. Their uncertainties are twice the final overall standard deviation of the mean and were estimated as outlined by Olofsson.19 The values for the standard molar enthalpies of formation of H2O(l) and CO2(g) at T = 298.15 K are −285.830 ± 0.042 and −393.51 ± 0.13 kJ·mol−1, respectively, taken from CODATA.20 The enthalpies of sublimation were deduced from the temperature dependence of the vapor pressure (Clausius− Clapeyron equation, eq 4): ln p = −B(T )−1 + A

(4)

This equation was fitted by the least-squares method to calculate the standard molar enthalpies of sublimation at the mean temperature, Tm, from B = ΔcdgHm°(Tm)/R. The molar enthalpies ΔcdgHm° at T = 298.15 K were computed by use of eq 5: g ° (T = 298.15 K) Δcd Hm g

III. COMPUTATIONAL METHODS Quantum chemical calculations were carried out with the Gaussian 09 package.16 The geometries of the compounds under investigation, and their most significant conformers, were optimized by use of the density functional B3LYP and M052X17 methods with 6-311++G(d,p) basis set without symmetry restrictions. Harmonic vibrational frequencies without scaling were calculated at the same theory level in all cases, to verify that all the stationary points are minima. The levels of theory employed in the present work are expected to provide consistent results for the considered reactions.18 Full computational details are given in Supporting Information.

° (Tm) = Δcd Hm +

298.15

∫T

m

[C °p ,m(g) − C °p ,m(cd)] dT

(5)

where Cp,m°(g) = f(T) was derived from computational data at the M05/6311++G(d,p) level and Cp,m°(cd) = f(T) was taken from the experimental DSC results. The standard molar enthalpies of sublimation and formation of the hydroxycinnamic acids studied, in condensed and gaseous states, at T = 298.15 K are given in Table 1. B. Structures and Thermochemistry Properties. The computed energies and enthalpies for the stable conformations of the molecules studied are described in detail in Supporting Information. All of them are considered as trans isomers, with dihedral angle θ(C1−C7−C8−C9) equal to ±180°. The dihedral angle ω(C7−C8−C9−O10) equal to either 0° or 180° defines syn or anti geometries, respectively. The orientation of R3, R4,

IV. RESULTS AND DISCUSSION A. Combustion Calorimetry and Knudsen Effusion Technique: Standard Molar Enthalpies of Formation In the Gas Phase, ΔfHm°(g). The results from combustion 2262



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methods such as “bond separation isodesmic reaction”9 (eq 6) and other isodesmic reactions.

and R5 substituents of the aromatic ring gives rise to several rotamers, whose arrangements are described by ϕ, β, and α dihedral angles related by rotation around the Cj−O bonds (j = 3, 4, 5) (Scheme 1).

(OCH3)m (OH)n ‐CA + (10 + m + n)methane → 5ethane + 4ethene + n methanol + m dimethyl ether + formaldehyde

(6)

where n and m are the numbers of hydroxyl and methoxyl substituents in the hydroxycinnamic acid molecule. 1. Cinnamic Acid, CA (R3 = R4 = R5 = H). We derived the enthalpy of formation in the gas phase of CA as ΔfHm°(g) = −229.8 ± 1.9 kJ·mol−1 (Table 1), using the experimental values of enthalpy of formation in condensed state, ΔfHm°(cd), and g enthalpy of sublimation, Δcd Hm°, reported by Pedley11and 21 Monte and Hillesheim, respectively. The theoretical estimations of ΔfHm°(g) by use of isodesmic reactions 6 (with m = 0, n = 1), 7, and 8 are rather close to the experimental one, with deviations less than 1 kJ·mol−1 for B3LYP (eq 6) and M05-2X theories (eqs 7 and 8). CA + 2methane → ethene + acetic acid + toluene

(7)

CA + methane → acetic acid + styrene

(8)

2. p-Coumaric Acid, 4-OH-CA (R3 = R5 = H, R4 = OH). Estimation of ΔfHm°(g) of 4-OH-CA by use of isodesmic reactions 6 (with m = 0, n = 2), 9, and 10, particularly with M05-2X theory, was in good agreement with the experimental value (deviation less than 2.9 kJ·mol−1).

Figure 1. Molecular geometry of the most stable trans-hydroxycinnamic acid conformers optimized at the M05-2X/6-311++G** level: CA, cinnamic acid (ω = 0°); 4-OH-CA, p-coumaric acid (ω = β = 0°); 3,4-diOH-CA, caffeic acid (ω = β = α = 0°); 4-OH-3-OCH3-CA, ferulic acid (ω = α = β = 0°); 3-OH-4-OCH3-CA, iso-ferulic acid (ω = β = 0°, α = 180°); and 3,5-diOCH3-4-OH-CA, sinapic acid (ω = α = 0°, β = ϕ = 180°).

(2‐, 3‐, or 4‐)OH‐CA + methane → CA + methanol

(2‐, 3‐, or 4‐)OH‐CA + benzene → CA + phenol

(9) (10)

The enthalpy of reaction 10 gives us information about the effect of introduction of an -OH group in CA, measured relative to the same effect on benzene. The experimental value ΔrHm° (10) = −0.9 ± 4.9 kJ·mol−1 indicates that the effect of p-OH introduction in CA is practically the same as that in benzene; therefore it suggests that -OH substituent on aromatic ring does not affect the propenoic side of p-coumaric acid. Thus we wondered whether o- or m-OH-substituted molecules show similar behavior.

All the most stable conformers obtained here (Figure 1) present a planar arrangement of the aromatic ring and the propenoic acid residue, with syn geometries. The rest of the conformers considered display a quite low enthalpy difference relative to the corresponding most stable one (less than 5 kJ·mol−1). The theoretical optimized geometries for the structures of these compounds, except 3,4-diOH-CA, are in good agreement with the available experimental XRD data (see Table S5 of Supporting Information). The case of 3,4-diOH-CA is interesting: in crystalline state this compound is formed by a nonchelated conformer (see 3,4-diOH-CAnoHB conformers in Supporting Information), which is 19−23 kJ·mol−1 less stable than the other 3,4-diOH-CA chelated conformers. So, in the condensed state, interactions such as intermolecular hydrogen bonds probably assist the presence of nonchelated molecules; while in the gas phase the 3,4-diOH-CA molecules retain their intramolecular hydrogen bonds. Thermochemical data on hydroxycinnamic acids are scarce. One of the reasons can be the imprecise experimental determination of enthalpy of sublimation, because many of these compounds sublimate at high temperatures with thermal decomposition (decarboxylation).23 In this context, we decided to check the experimental enthalpy of formation of the species reported in this work [(OCH3)m(OH)n-CA] using theoretical

Figure 2. Molecular geometries of the most stable conformers of ocoumaric acid, 2-OH-CA (ω = 0°, dihedral angle C1−C6−O−H = 180°), and m-coumaric acid, 3-OH-CA (ω = α = 0°), optimized at the M05-2X/6-311++G** level.

3. o- (or m-) Coumaric Acids, 2- (or 3-) OH-CA. The ΔfHm°(g) values of these isomers are not available. Thus, we can determine them by use of homodesmotic reactions 10 and 11. The geometry of the most stable conformers is depicted in Figure 2. We can see that the preferential orientation of -OH moiety in 3-OH-CA (α = 0°) is the same that in 4-OH-CA (β = 0°), while on the contrary, it is the opposite (dihedral angle C1−C6−O−H = 180°) in 2-OH-CA. On the other hand, it is 2263



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Scheme 2. Enthalpic Increments (ΔΔfHm°) Associated with Introduction of Hydroxyl Groups in Benzene and CA Moleculesa

important to remark that the less stable 2-OH-CA conformers (with enthalpic differences of almost 7 kJ·mol−1) have a nonplanar geometry, reflected in considerable deviations in the planarity of θ(C1−C6−O−H) dihedral angles. 4‐OH‐CA → 2‐(or 3‐)OH‐CA

(11)

The theoretical average values (obtained with B3LYP and M05-2X theories) of reaction enthalpies ΔrHm° (10) and ΔrHm° (11) for 2-OH-CA were −1.6 and −4.0 kJ·mol−1, respectively. For 3-OH-CA, the values were −0.8 and −3.2 kJ·mol−1, respectively. From these results we can deduce that (i) the position of -OH substituent on the benzenic ring of coumaric acid practically does not affect its propenoic acid end and (ii) the average values of ΔfHm°(g) for 2- and 3-OH-CA are respectively −405.6 and −406.4 kJ·mol−1, both with an estimated uncertainty of ±4.4 kJ·mol−1. 4. Caffeic Acid, 3,4-diOH-CA (R4 = R5 = OH, R3 = H). In the experimental determination of this ΔfHm°(g) value, we have considered the experimental value of enthalpy of sublimation, g Δcd Hm° (T = 418.5 K) = 167.3 ± 5.2 kJ·mol−1 as reported by Chen and co-workers.22 By means of eq 5, we determined the enthalpy of sublimation at T = 298.15 K (see Table 1). The theoretical ΔfHm°(g) value of caffeic acid estimated by eq 6 (with m = 0, n = 3) is close to the experimental one only at the B3LYP level (deviation of 3.6 kJ·mol−1), whereas calculations by isodesmic reactions 12 or 13 give values with deviation less than 2.7 kJ·mol−1 for both B3LYP and M05-2X theories.

a

All values are given in kilojoules per mole; those in parentheses are ΔfHm°(g) values.

5. Ferulic Acid, 4-OH-3-OCH3-CA (R5 = OCH3, R4 = OH, R3 = H). We have considered only four of the most significant stable conformers with relative stabilities between 1 and 5 kJ·mol−1 (see Supporting Information). The contribution to the population of the other less stable conformers is rather small, because they display high energy differences relative to the most stable conformer (between 18 and 56 kJ·mol−1).24 We found enthalpy differences between 21 and 26 kJ·mol−1 for the significant nonchelated conformers (see Supporting Information). The standard molar enthalpy of formation of 4-OH-3-OCH3CA estimated from eq 6 (with m = 1, n = 2) and homodesmotic reactions 14 and 15 is in good agreement with the experimental one, particularly at the M05-2X level (deviation less than 3.9 kJ·mol−1). The ΔfHm°(g) values for 3-methoxycinnamic acid (−381.0 ± 3.9 kJ·mol−1) and 2-methoxyphenol (−246.1 ± 1.9 kJ·mol−1) were taken from Matos et al.25,26

Purely experimental values of ΔrHm° (12) and ΔrHm° (13) are 0.1 ± 6.5 and 0.9 ± 7.6 kJ·mol−1, respectively. These values indicate the lack of interaction between the propenoic fragment and -OH substituents in 3,4-diOH-CA. Therefore, this result would explain why (i) the increments in the stability (ΔΔfHm°) due to successive introduction of -OH groups in CA molecule to yield caffeic acid are practically the same as the corresponding increments in benzene to yield catechol (shown in Scheme 2) and (ii) the geometrical parameters of the -OH groups in 3,4-diOH-CA, forming an intramolecular hydrogen bond (HB), are close to the corresponding geometrical parameters of catechol: donor−acceptor distances d(Hd···Oa) = 2.075 Å and d(O···Oa) = 2.624 Å and angle O− Hd···Oa = 108.0°. Even more, the enthalpy differences between chelated and nonchelated structures of 3,4-diOH-CA and catechol are the same (19 kJ·mol−1) and can be taken as HB measurements (see Supporting Information).

The values of ΔrHm° (14) = 7.9 ± 6.3 kJ·mol−1, ΔrHm° (15) = 6.4 ± 7.0 kJ·mol−1, and ΔrHm° (16) = 7.3 ± 8.4 kJ·mol−1, might indicate, within their associated uncertainties, a small or even negligible interaction between the propenoic fragment and -OCH3 and -OH groups in the disubstituted 4-OH-3-OCH3CA. This fact is consistent with the following results: (i) The enthalpies of reactions reactions 14 and 15 and 16 are related to the enthalpic increments (ΔΔfHm°) associated with the insertion of -OH or -OCH3 in both 2264



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Scheme 3. Enthalpic Increments (ΔΔfHm°) Associated with Introduction of Hydroxyl and Methoxyl Groups in Benzene and CA Moleculesa

a

All values are given in kJ·mol−1; those in parentheses are ΔfHm°(g) values.

they suggest that a certain character of HB interaction remains in the molecules studied, in spite of the lack of bond path. 6. iso-Ferulic Acid, 3-OH-4-OCH3-CA (R5 = OH, R4 = OCH3, R3 = H). The experimental ΔfHm°(g) value of this molecule is not available. Nevertheless, by use of the homodesmotic reaction 4-OH-3-OCH3-CA → 3-OH-4-OCH3-CA at the M052X level of theory, we found that the enthalpy of formation of 3-OH-4-OCH3-CA is ΔfHm°(g) = −565.2 ± 5.7 kJ·mol−1 (with the same uncertainty as ferulic acid). Given that both isomers have similar stabilities, we deduce that the interaction among propenoic, -OCH3, and -OH groups is energetically similar. 7. Sinapic Acid, 3,5-diOCH3-4-OH-CA (R5 = R3 = OCH3, R4 = OH). The experimental enthalpy of this molecule is not available in the literature. However we can determine a confidence value by combining theoretical and experimental data by use of eq 6 (with m = 2, n = 2) and the following isodesmic reactions:

CA and benzene derivatives (see Scheme 3). Considering the uncertainties assigned, the ΔΔfHm° values determined for -OH insertion in 3-methoxycinnamic acid and for -OCH3 insertion in 4-OH-CA, to yield ferulic acid, are close to the corresponding ΔΔfHm° values obtained for the introduction of the same groups in anisole and phenol, to yield 2-methoxyphenol. (ii) In 4-OH-3-OCH3-CA, the -OCH3 and -OH groups form an HB, whose geometrical and energetic parameters are very close to the corresponding parameters of 2methoxyphenol (see Supporting Information): donor− acceptor distances d(Hd···Oa) = 2.075 Å and d(O···Oa) = 2.624 Å and angle O−Hd···Oa = 114.4°, or to the enthalpy difference among chelated and nonchelated structures (average of 22.5 kJ·mol−1, taking into account the population of the conformers considered). It is interesting to mention that the presence of the HB in catechol and 2-methoxyphenol has been investigated both experimentally and computationally at various levels.27−29 Frequency red shifts displayed by O−H stretching bands and geometrical considerations (donor−acceptor distances, etc.) were used as evidence of HB presence in these molecules. However, the computational results (using quantum theory of atoms in molecules, QTAIM) of Mosquera and co-workers30 questioned the nature of such HB interactions because they found that catechol29 and caffeic, ferulic, iso-ferulic, and sinapic acids do not present any bond paths assignable to an HB, even though these authors consider that HB bond paths appear after small distortions in the optimized geometries and also with certain O−H bond dissociation enthalpy considerations. Thus, 2265



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(CA), o-, m-, and p-coumaric acid (2-, 3-, and 4-OH-CA), caffeic acid (3,4-diOH-CA), ferulic acid (4-OH-3-OCH3-CA), iso-ferulic acid (3-OH-4-OCH3-CA), and sinapic acid (3,5diOCH3-4-OH-CA) are reported and discussed in this work. Experimental values of the standard molar enthalpy of formation in the gas phase, ΔfHm°(g), of CA (updated), 4-OHCA, 3,4-diOH-CA, and 4-OH-3-OCH3-CA have allowed estimation of ΔfHm°(g) values of 3-OH-4-OCH3-CA, 3,5diOCH3-4-OH-CA, and o- and m-coumaric acids, by use of isodesmic reactions. In general, the most reliable values of ΔfHm°(g), very close to the experimental ones, were obtained by means of the M05-2X theory. B3LYP theory was also adequate in several cases, particularly for “bond separation isodesmic reactions”. Thus, the use of M05-2X methodology could be considered as a significantly improved theoretical method to predict values of ΔfHm°(g).10 Interaction between the neighboring -OCH3 and/or -OH groups in hydroxycinnamic acids takes place without a significant influence of the propenoic fragment. Our results, from a purely energetic point of view, are consistent with the purely computational results approached by Mosquera and coworkers,30 whose QTAIM analysis indicates that the inclusion of a π-donor substituent (-OH or -OCH3) does not alter the πelectron atomic populations of the propenoic fragment in the hydroxycinnamic acids. The insignificant influence of propenoic fragment on -OCH3 and -OH groups in hydroxycinnamic acids is also seen for methoxycinnamic acids, inasmuch as Matos et al.25 deduced, from their experimental results; neither steric nor electronic interactions occur between propenoic fragment and the -OCH3 groups in these molecules.

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The ΔfHm°(g) value for 2,6-dimethoxyphenol (−381.7 ± 1.9 kJ·mol−1) was taken from Matos et al.26 The weighted average value of enthalpy of formation of sinapic acid, obtained at the M05-2X level, is ⟨ΔfHm°(g)⟩ = −698.8 ± 4.1 kJ·mol−1, where the uncertainty is assumed to be twice the weighted standard deviation (see Supporting Information). It is important to mention that the standard deviation obtained with B3LYP theory was almost twice as large as that for M05-2X. The geometry of the most stable conformer is depicted in Figure 1. The -OCH3 and -OH groups, as in ferulic or isoferulic molecules, form an HB whose geometrical parameters, d(Hd···Oa) = 2.084 Å, d(O···Oa) = 2.631 Å, and angle O− Hd···Oa = 114.2°, are very close to the corresponding parameters of 2,6-dimethoxyphenol (see Supporting Information). The insignificant interaction between propenoic fragment and -OCH3 and -OH groups can be confirmed by the following results in sinapic acid: (i) the calculated value of ΔrHm° (eq 20) = 4.7 ± 5.0 kJ·mol−1 indicates that the effect of the introduction of -OH and -OCH3 groups in CA is practically the same, within the assigned uncertainty, as in benzene; and (ii) the ΔΔfHm° value due to insertion of -OCH3 group in ferulic acid to yield sinapic acid is close to the corresponding ΔΔfHm° value obtained for introduction of the same group in 2-methoxyphenol to yield 2,6-dimethoxyphenol (see Scheme 3).

ASSOCIATED CONTENT

S Supporting Information *

Table of experimental ΔfHm°(g) values used for reference compounds of isodesmic reactions; DSC measurements, fusion points, enthalpy of fusion, and molar heat capacities; combustion calorimetry; sublimation and Knudsen effusion technique; computational results; experimental XRD data and DFT calculated geometrical parameters; computational results of nonchelated caffeic acids (3,4-diOH-CAnoHB) and catechol; computational results of 2-methoxyphenol and ferulic acids nonchelated (4-OH-3-OCH3-CAnoHB); geometrical parameters of 2,6-dimethoxyphenol; and enthalpy of formation of sinapic acid. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

́ This work is dedicated to Professor Victor Latorre (Faculdad de . Ciencias, Universidad Nacional de Ingenieria,́ Lima-Perú). The support of the Spanish MICINN Project CTQ2009-13652 is gratefully acknowledged. Work by A.C. and A.G. was carried out under JAE-Doc and JAE-predoc contracts with CSIC.

V. SUMMARY AND CONCLUSIONS Consistent experimental and theoretical investigation on the structural and thermodynamic properties of trans-cinnamic acid 2266



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