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J. Phys. Chem. B 2009, 113, 11216–11221
Energetics of Coumarin and Chromone M. Agostinha R. Matos,*,† Clara C. S. Sousa,† Margarida S. Miranda,† Victor M. F. Morais,†,‡ and Joel F. Liebman§ Centro de InVestigac¸a˜o em Quı´mica, Departamento de Quı´mica, Faculdade de Cieˆncias da UniVersidade do Porto, Rua do Campo Alegre, 687, P-4169-007 Porto, Portugal, Instituto de Cieˆncias Biome´dicas Abel Salazar, ICBAS, UniVersidade do Porto, P-4099-003 Porto, Portugal, and Department of Chemistry and Biochemistry, UniVersity of Maryland, Baltimore County, Baltimore, Maryland 21250 ReceiVed: March 25, 2009; ReVised Manuscript ReceiVed: May 22, 2009
Condensed phase standard (p° ) 0.1 MPa) molar enthalpies of formation for coumarin and chromone were derived from the standard molar enthalpies of combustion, in oxygen, at T ) 298.15 K, measured by static bomb combustion calorimetry. The standard molar enthalpies of sublimation, at T ) 298.15 K, were measured by Calvet microcalorimetry. Combining these values, the following enthalpies of formation in the gas phase, at T ) 298.15 K, were then derived: coumarin, -(163.4 ( 3.3) kJ · mol-1, and chromone, -(126.1 ( 2.5) kJ · mol-1. The temperatures of fusion, Tfusion, and fusion enthalpies, at T ) Tfusion, were also reported. Additionally, theoretical calculations were done using different methods: DFT/B3LYP, MCCM (MC-UT/3 and MC-QCISD/3), and also the more accurate G3MP2 method. Good agreement between experimental and theoretical data was achieved. Some correlations between structure and energy were also made, and the aromaticity of the compounds was evaluated by the nucleus independent chemical shifts (NICS). 1. Introduction In nature, benzopyran analogues are widespread, and many of them have interesting biological and physical properties.1 Coumarin was first identified in 1820 and was first synthesized in the laboratory in 1868.2 It is a pleasant smelling compound which gives a characteristic odor to hay. Other simple coumarins also possess characteristic smells sometimes exploited in perfumery.3 Coumarin derivatives also have diverse biological properties, such as enzyme inhibition, hypotoxicity, as well as carcinogenicity or anticoagulant or antibiotic action.4,5 Some are also dyes owing their success to their efficient light emission properties, high stability, and ease of synthesis.6 Chromone and its derivatives have become interesting targets in organic synthesis due to their high importance in biological systems. They are widespread in nature, particularly in flowers as pigments, and have applications in the pharmaceutical industry and cosmetics. Some of them have anti-inflammatory properties, act as skin protectors, and have laxative effects.7-9 In this work we report the experimental standard molar enthalpies of formation in the gas phase, at T ) 298.15 K, of the two isomers, coumarin and chromone, and the respective values estimated by computational methods. 2. Experimental Details The studied compounds {coumarin [CAS 91-64-5] and chromone [CAS 491-38-3]} were obtained from Aldrich Chemical Co. All the samples were purified by repeated sublimation under reduced pressure before the experimental studies. The purity and enthapies of fusion, at T ) Tfusion (Table * Corresponding author. E-mail:
[email protected]. Phone: +351 22 0402 517. Fax: +351 22 0402 522. † Centro de Investigac¸a˜o em Quı´mica, Departamento de Quı´mica, Faculdade de Cieˆncias da Universidade do Porto. ‡ Instituto de Cieˆncias Biome´dicas Abel Salazar, ICBAS, Universidade do Porto. § University of Maryland.
TABLE 1: Temperatures of Fusion, Tfus, Enthalpies of l Fusion, ∆cr H°m(Tfus), and Purity of Coumarin and Chromone compd
Tfus/K
l ∆cr H°m(Tfus)/kJ · mol-1
purity
coumarin chromone
342.30 ( 0.04 329.89 ( 0.05
18.63 ( 0.08 15.44 ( 0.12
99.94 ( 0.02 99.97 ( 0.01
1), were derived from DSC (Setaram DSC 141) analysis by a fractional fusion technique.10 The samples, hermetically sealed in stainless steel crucibles, were heated at 1.67 × 10-2 K · s-1. The temperature scale of the calorimeter was calibrated by measuring the melting temperatures of three high purity reference materials (naphthalene, benzoic acid, and indium),11 and its power scale was calibrated with high-purity indium (mass fraction >0.99999). The purity of the samples was also confirmed through the carbon dioxide gravimetry results. The average ratios, together with the standard deviation of the mean, of the mass of carbon dioxide recovered after each combustion experiment to that calculated from the mass of sample were as follows: coumarin (0.9997 ( 0.0005) and chromone (0.9997 ( 0.0002). The densities of the samples estimated from the mass and the dimensions of pellets of the crystalline compounds are as follows: coumarin (0.94 g · cm-3) and chromone (1.14 g · cm-3). The standard molar enthalpies of the compounds in the condensed phase were obtained from combustion calorimetry with a static bomb calorimeter. The apparatus and technique have been described previously.12,13 Benzoic acid (Bureau of Analyzed Samples, Thermochemical Standard, BCS-CRM-190 p) was used for calibration of the bomb. Its massic energy of combustion is ∆cu° ) -(26435.1 ( 3.5) J · g-1 under certificate conditions. The calibration results were corrected to give the energy equivalent εcal corresponding to the average mass of water added to the calorimeter, 3119.6 g. From six independent calibration experiments performed, εcal ) (16004.8 ( 1.6) J · K-1, where the uncertainty quoted is the standard deviation of the mean.
10.1021/jp9026942 CCC: $40.75 2009 American Chemical Society Published on Web 07/20/2009
Energetics of Coumarin and Chromone
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The compounds were burnt in pellet form. Coumarin, due to its volatility, was enclosed in polyester bags made of Melinex, using the technique described by Skinner and Snelson,14 who determined the specific energy of combustion of dry Melinex as ∆cu° ) -(22902 ( 5) J · g-1. This value was confirmed in our laboratory. The mass of Melinex used in each experiment was corrected for the mass fraction of water (0.0032), and the mass of carbon dioxide produced from it was calculated using the factor previously reported.14 Combustion experiments were performed in oxygen at p ) 3.04 MPa, with 1.00 cm3 of water added to the bomb: ∆UΣ is the correction to the standard state. For the cotton-thread fuse, empirical formula CH1.686O0.843, ∆cu° ) -16250 J · g-1.15 This value has been confirmed in our laboratory. The corrections for nitric acid formation ∆U(HNO3) were based on -59.7 kJ · mol-1 16 for the molar energy of formation of 0.1 mol · dm-3 HNO3(aq) from N2, O2, and H2O(l). The mass of compound, m(compound), used in each experiment was determined from the total mass of carbon dioxide, m(CO2, total), produced after allowance for that formed from the cotton thread fuse and Melinex. An estimated pressure coefficient of specific energy (∂u/∂p)T ) -0.2 J · g-1 · MPa-1 at T ) 298.15 K, a typical value for most organic compounds, was assumed.17 As samples were ignited at T ) 298.15 K,
∆U(IBP) ) -{εcal + ∆m(H2O) cp(H2O, l) + εf}∆Tad + ∆Uign (1) where ∆U(IBP) is the energy associated with the isothermal bomb process, εf is the energy of the bomb contents after ignition, and ∆Tad is the adiabatic temperature rise calculated using the program LABTERMO.18 For each compound, the corrections to the standard state, ∆UΣ, to derive the standard massic energy of combustion, ∆cu°, were made by the procedure given by Hubbard et al.19 The atomic weights of the elements were those recommended by the IUPAC commission in 2005.20 The standard molar enthalpies of sublimation were measured using the “vacuum sublimation” drop microcalorimetric method.21 Samples, of about 3-5 mg of the compounds contained in thin glass capillary tubes sealed at one end, were dropped, at room temperature, into the hot vessel in a hightemperature Calvet microcalorimeter held at a convenient temperature (365 K for coumarin and 375 K for chromone) and then removed from the hot zone by vacuum sublimation. The thermal corrections for the glass capillary tubes were determined in separate experiments and were minimized, as far as possible, by dropping tubes of nearly equal mass into each of the twin calorimeter cells. The microcalorimeter (Calvet High Temperature Microcalorimeter, SETARAM HT 1000) was calibrated in situ for these measurements using the reported standard molar enthalpies of sublimation of naphthalene.22 From six independent experiments we have obtained a mean value for the observed standard molar enthalpy of sublimation g,T H°m, which was then corrected to of each compound, ∆cr,298.15K T H°m(g), using the equation: T ) 298.15 K, ∆298.15K T ∆298.15K H°m(g) )
T C°p,m(g) dT ∫298.15K
(2)
where T is the temperature of the hot reaction vessel and C°p,m(g) is the molar heat capacity of the compounds in the gas phase and was obtained from statistical thermodynamics using the
vibrational frequencies obtained from the DFT calculations with the B3LYP functional and the 6-31G* basis set. Coumarin
C°p,m(g)/(J · mol-1 · K-1) ) - 0.000400(T/K)2 + 0.721(T/K) - 43.617 (3) Chromone
C°p,m(g)/(J · mol-1 · K-1) ) - 0.000389(T/K)2 + 0.712(T/K) - 42.833 (4) As may be expected for a pair of isomers, the various coefficients and constant term are very much the same. The enthalpies of formation in the gas phase of the compounds, ∆fH°m(g), at T ) 298.15 K, were determined from the experimental values of its standard enthalpies of formation in the condensed state, ∆fH°m(cr), and the standard phase change g enthalpies, ∆cr H°m. 3. Computational Details The geometries of all molecules have been fully optimized using density functional theory (DFT) with the Becke 3-parameter hybrid exchange23 and the Lee-Yang-Parr24 correlation density functionals (B3LYP) and the Pople’s split-valence 6-31G* extended basis set.25 The optimum structures so obtained where further certified as true minima by constructing and diagonalizing the corresponding Cartesian Hessian matrix, this procedure providing also the harmonic vibrational frequencies, which, after properly scaled by the recommended scaling factor 0.9614,26 allow reliable calculations of the thermal corrections to the molecular energy. We have further refined the optimum structures by reoptimizing them using the same methodology with the Pople’s split-valence 6-311G** extended basis set.27 These final optimized structures where then used to perform single point DFT calculations with the cc-pVTZ basis set28 and also energy calculations based on more accurate correlated computational techniques of the MCCM/3 suite.29,30 In addition, we have also conducted calculations using the G3MP2 method.31 This computational methodology relies on much less drastical approximations than the methods of the MCCM/3 suite, thus being likely to provide consistently more reliable energetic estimates. However, this methodology still suffers from the drawback of requiring computational resources on a scale which are only compatible with moderately sized molecules. Thus, one of our aims when conducting these calculations is also to certify the more computationally more modest methods of the MCCM/3 suite, which can become, on the other hand, the only reliable calculations to adopt when dealing with larger systems. All the geometry optimizations, vibrational analysis, and single point calculations have been performed using the U.K. version of the program GAMESS.32,33 The MCCM/3 series of calculations have been performed using the MLGAUSS program version 2.0,34 which relies on the Gaussian 03 series of programs.35 The Nucleus Independent Chemical Shifts (NICS) values were calculated using B3LYP/6-311G** wave functions at the B3LYP/6-311G** geometries. The methodology used was developed by Schleyer and his co-workers as a means of providing useful aromaticity indices.36 Two different values were calculated for each ring and each molecule: one at the geometrical center of the ring (i.e., the point whose coordinates
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TABLE 2: Typical Combustion Experiments, at T ) 298.15 K m(CO2, total)/g m(cpd)/g m(fuse)/g m(Melinex)/g ∆Tad/K εf/(J · K-1) ∆m(H2O)/g -∆U(IBP)/J ∆U(fuse)/J ∆U(Melinex)/J ∆U(HNO3)/J ∆U(ign)/J ∆UΣ/J -∆cu°/J · g-1
coumarin
chromone
1.59011 0.53350 0.00402 0.06012 1.0341 15.67 0.2 16566.33 65.28 1376.85 2.78 1.19 12.12 28321.09
1.71030 0.62953 0.00254
g compd -∆fH°m(cr)/kJ · mol-1 ∆cr H°m/kJ · mol-1 -∆fH°m(g)/kJ · mol-1
coumarin chromone
1.1284 15.81 0.0 18076.60 41.25
benzene coumarin chromone
1.47 1.06 13.04 28625.86
28648.16 28632.26 28625.86 28621.09 28624.06 28656.22 28653.62
28637.3 ( 5.6
TABLE 4: Derived Standard (p° ) 0.1 MPa) Molar Values, at T ) 298.15 K compd coumarin chromone
-∆cU°m/kJ · mol-1 -∆cH°m/kJ · mol-1 -∆fH°m(cr)/kJ · mol-1 4139.1 ( 1.7 4185.2 ( 2.0
4140.3 ( 1.7 4186.5 ( 2.0
163.4 ( 3.3 126.1 ( 2.5
pyrone ring
NICS(0)
NICS(1.0)
NICS(0)
NICS(1.0)
-9.7 -9.40 -10.27
-11.5 -10.69 -11.30
-0.42 0.41
-3.24 -3.12
a In parentheses we show the total contribution from donoracceptor interactions to the NICS values.
chromone
-〈∆cu°〉/J · g-1 28321.9 ( 3.2
95.4 ( 2.6 86.5 ( 1.1
benzene ring
-∆cu°/J · g-1 28339.51 28310.09 28323.90 28318.35 28317.95 28321.09 28317.85 28313.42 28334.90
258.8 ( 2.1 212.6 ( 2.2
TABLE 6: Nucleus Independent Chemical Shifts (ppm)a
TABLE 3: Individual Values of the Massic Energy of Combustion, at T ) 298.15 K coumarin
TABLE 5: Derived Standard (p° ) 0.1 MPa) Molar Enthalpies of Formation, at T ) 298.15 K, of Coumarin and Chromone
258.8 ( 2.1 212.6 ( 2.2
are the nonweighted mean of the homologous coordinates of the heavy atoms of the rings), denoted NICS(0), and 1.0 Å above the center of the ring, denoted NICS(1.0). The calculation of NICS values has been performed with the Gaussian 03 series of programs.35 4. Results 4.1. Experimental Results. The enthalpies of fusion, determined at T ) Tfus, the fusion temperatures, and purity were obtained from the DSC experiments. The recorded thermograms did not show any phase transition between 298.15 K and the melting temperature of the compounds. The values are reported in Table 1 with reference to the mean values of six independent experiments on fresh samples, and the uncertainties are twice the standard deviation of the mean. Table 2 lists, for the two isomers, a typical combustion experiment corresponding to the combustion reaction
In accordance with customary thermochemical practice,37 the uncertainty assigned to the standard molar enthalpies of combustion is twice the overall standard deviation of the mean and includes the uncertainties in calibration and in the values of auxiliary quantities used. In order to derive ∆fH°m(cr) from ∆cH°m(cr), the standard molar enthalpies of formation of H2O(l) and CO2(g), at T ) 298.15 K, -(285.830 ( 0.042) kJ · mol-1 38 and -(393.51 ( 0.13) kJ · mol-1,38 respectively, were used. The standard molar enthalpies of sublimation of the compounds, at the temperature T ) 298.15 K, were obtained from at least six independent microcalorimetric experiments. The uncertainty of the results is twice the standard deviation of the mean. From the values for the standard molar enthalpies of formation and sublimation of the crystalline compounds, the values of the standard molar enthalpies in the gaseous phase were derived. These results are summarized in Table 5. 4.2. Computational Results. As would be expected for systems mainly consisting of sp2-hybridized fragments, both coumarin and chromone are found to adopt completely planar structures at their most stable molecular geometries. Indeed, it is this planar conformation that favors the occurrence of stabilizing extended electronic delocalization involving all those fragments, which, when involving cyclic structures, evidences the peculiar stabilizing effect we generally call aromaticity. Energetically, both our calculations and our experimental findings predict coumarin to be more stable than chromone (by about 37.3 ( 4.1 kJ · mol-1), a feature which results from the peculiar structure of the former isomer which involves the much more stable OsCdO fragment, which has already been identified as determining also the relative stability of chromanone, isochromanone, and dihydrocoumarin.39 We also recall this ester fragment has considerable stabilization energy, identified as resonance energy and comparable to that of NsCdO, i.e., the highly stabilized isoelectronic amide group.40,41 In fact, for the latter systems, we also found that the isomers containing the OsCdO fragment (isochromanone and dihydrocoumarin) are more stable than chromanon, and that dihydrocoumarin, which is the saturated analogue of coumarin, is
C9H6O2 (cr) + 19/2O2 (g) f 9CO2 (g) + 3H2O (l)
(5) Table 3 lists the individual values for the combustion experiments of the studied compounds, and Table 4 shows the derived standard molar energies and enthalpies of combustion, ∆cU°m(cr) and ∆cH°m(cr), and the standard molar enthalpies of 0 (cr), at T ) 298.15 K. formation in the crystalline phase, ∆fHm
Figure 1. Coumarin (I) and chromone (II).
Energetics of Coumarin and Chromone
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TABLE 7: Electronic Energies, DFT and MCCM, and Thermal Corrections to T ) 298.15 Ka compd
EB3LYP/6-311G**
EB3LYP/cc-pVTZ
EMC-UT/3
EMC-QCISD/3
HG3MP2 (T ) 298.15 K)
TCEB3LYP/6-31G*
coumarin chromone chromanone dihydrocoumarin R-tetralone cyclohexene dihydro-2H-pyran indene cyclohexane indane
-497.154498 -497.140248 -498.354741 -498.370265 -462.447184 -234.711494 -270.621515 -347.852017 -235.947115 -349.078178
-497.209261 -497.194257 -498.407293 -498.423653 -462.494829 -234.734012 -270.650623 -347.890114 -235.969805 -349.115138
-496.363935 -496.350032 -497.567733 -497.584987 -461.685403 -234.311319 -270.195001 -347.245356 -235.543802 -348.472252
-496.371079 -496.356733 -497.573965 -497.591277 -461.689668 -234.312210 -270.197607 -347.248729 -235.544528 -348.475258
-496.265407 -496.250239 -497.443890 -497.461623 -461.544546 -234.186286 -270.086380 -374.140397 -235.395703 -348.343286
0.130701 0.131089 0.154020 0.153806 0.177571 0.146976 0.123831 0.142084 0.170406 0.165800
a
298.15K All energies are in a.u (1EH ) 2625.50184 kJ · mol-1). TCEB3LYP/6-31G* ) Etrans + Erot + Ezp + ∆0K Evib.
TABLE 8: Theoretical Estimates of the Standard Enthalpies of Formation in the Gas Phase at T ) 298.15 K of Coumarin and Chromone - ∆fH°m(g)/kJ · mol-1 DFT/B3LYP compd
R
6-311G**
cc-pVTZ
MC-UT/3
MC-QCISD/3
G3MP2
exp
coumarin
IA IB At IIA IIB Ata
181.8 171.5 80.5 140.3 133.5 42.0
185.8 172.1 129.2 144.5 133.4 88.7
159.7 159.5 140.8 123.6 119.8 103.3
161.5 160.8 143.6 124.4 121.3 104.9
164.2 162.3 173.8 127.5 124.2 134.0
163.4 ( 3.3
chromone
a
126.1 ( 2.5
Atsatomization reactions.
the most stable isomer. Considering only the homologous pairs of both classes of compounds, we can observe that dihydrocoumarin is 43.4 ( 3.3 kJ · mol-1 more stable than chromanone while coumarin is only 37.3 ( 4.1 kJ · mol-1 more stable than chromone, thus evidencing an attenuation of the stabilizing capacity of the OsCdO fragment by somewhat more than 6 kJ · mol-1. We must recall, at first, that the enhanced stability afforded by that fragment can evidently be associated with the possibility of involvement of the lone-electronic pairs of both oxygen atoms in conjugative or hyperconjugative mutual interactions, which are especially important for these isomers, since the OsCdO fragment can directly interact with the contiguous aromatic ring, thus enhancing the extended electronic delocalization. Fragmentation of this group thus results in the observed stability differences, but the destabilizing effect is less pronounced for the pair coumarin/chromone, since, in the latter system, the intercalation of a CdC double bond in the pyrone ring between both oxygen atoms still tends to allow the mutual interactions involving the respective π-lone electronic pairs, thus avoiding the destabilization becoming so severe for these systems. These interactions involving both π-lone electronic pairs through the CdC double bond should be evidenced by some aromatic character in the pyrone ring of the coumarin, while no aromatic character should be evidenced by the homologous ring of dihydrocoumarin. In fact, nucleus independent chemical shifts (NICS) calculated from the corresponding B3LYP/6-311G** wave functions of both systems indicate that the NICS values at the center of the pyrone ring of chromanone and 1.0 Å above it are, respectively, +2.49 ppm and -0.16 ppm, while the corresponding values for dihydrocoumarin are +0.45 ppm and -0.73 ppm, respectively. These findings thus indicate essentially no aromatic character for both systems, even though the out of plane NICS value at the pyrone ring of dihydrocoumarin is somewhat more negative (the ring is consequently somewhat more aromatic), which is certainly a consequence of the electronic interactions involving the loneelectronic pairs of both oxygen atoms integrating the fragment
OsCdO. In contrast, the NICS values for chromone (and also for coumarin) reported in Table 6 indicate a moderate aromatic character of the pyrone ring of both systems, as is likewise shown for the monocyclic 2- and 4-pyrones as well. How aromatic are these pyrones? Enthalpy of formation data for these monocyclic pyrones, or any substituted derivative thereof, appear to be absent:42 while the isomerization enthalpy interrelating 4-methoxy-6-methyl-2-pyrone and 2-methoxy-6-methyl-4-pyrone has been measured,43 the presence of the exocyclic methoxy groups confounds comparison, as we have insufficient thermochemical knowledge of enol ethers and dialkoxyalkenes, as well as with pyrones of any type. Total energies, identified by the subscripts B3LYP/6-311G**, B3LYP/cc-pVTZ, MC-UT/3, and MC-QCISD/3, as well as thermal corrections, TCE/6-31G*, are reported in Table 7 for the studied compounds. We also report in that table the absolute enthalpies at 298.15 K, obtained from the G3MP2 calculations (in the column HG3MP2(T ) 298.15 K)). In order to estimate the enthalpies of formation of the systems from the calculated energies, we used the following set of isodesmic/homodesmic reactions involving auxiliary systems whose thermochemical properties are well established experimentally.35-38 The optimum geometries, the energies, and the thermal corrections for all the auxiliary molecules have also been obtained using the same procedures as described above. Both chromone and coumarin had been experimentally studied previously,44,45 but the obtained enthalpies of formation differ considerably for the present measurements. Indeed, the earlier obtained enthalpies of formation of -176.8 ( 1.8 kJ · mol-1 for coumarin and of -148.5 ( 2.9 kJ · mol-1 for chromone are in disagreement by as much as 13.4 kJ · mol-1 (coumarin) and 22.4 kJ · mol-1 (chromone), with both being excessively stable as compared with the present work. As can be seen from the results in Table 8, our calculations describe very accurately our own experimental data; indeed, with the exception of the results obtained from reaction IIB and
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Figure 2. Isodesmic reactions used in the calculation of the enthalpies of formation of coumarin (IA and IB) and chromone (IIA and IIB).
from MC-UT/3 and MC-QCISD/3 calculations, the most accurate composite methods (MC-UT/3, MC-QCISD/3, and G3MP2) provide estimates of the enthaply of formation of these compounds with deviations from the experimental data which do not exceed 4 kJ · mol-1 (i.e., we attain the standard of “chemical accuracy”). In addition, for the most accurate G3MP2 calculations, even the results obtained from the atomization processes are in good agreement with our experimental data, with deviations not larger than 10.5 kJ · mol-1, a remarkable result for non isodesmic reactions. Additional corroboration of our suggested values is that we find a 38.3 ( 4.1 kJ mol-1 difference between the enthalpies of formation of coumarin and chromone, as opposed to the earlier 28.3 ( 3.4 kJ · mol-1 but in agreement with the ca. 40 kJ · mol-1 difference found by our various calculational approaches. Recently, the enthalpies of combustion and formation were reported46 for 3-formylchromone and two alkyl derivatives. As these results apply only to the solid phase, we are loathe to attempt to use them in any isodesmic reaction, such as
chromone + benzaldehyde f 3-formylchromone + benzene as part of any method for the comparison of the competing values for the enthalpies of formation of the unsubstituted chromone. Thus, our very accurate computational estimates correctly describe our experimental data and, on the other hand, crudely diverge clearly from the earlier experimental data of Sabbah and Watik, with deviations of 16 kJ · mol-1 (coumarin) and 27.2 kJ · mol-1 (chromone). Finally, we feel confident to propose our new experimental data as the most reliable ultimate thermochemical data for coumarin and chromone. 5. Brief Thermochemical Discussion of the Aromaticity of Coumarin and Also Chromone In the above we discussed the aromaticity of coumarin within the context of NICS values. As thermochemical analysis reflects
our predilections more than from that from magnetic resonance derived reasoning, we now indulge our interest in the former by a brief discussion of precedent models from the literature. We might think that both species should be rather aromatic, since they are π-isoelectronic or isoconjugate to the aromatic hydrocarbon, naphthalene, much as furan is related to benzene. The first makes use of hydrogenation enthalpies: what are the enthalpies of hydrogenation of coumarin and chromone to their dihydrocounterparts? From ref 44 we find the enthalpies of formation of gaseous dihydrocoumarin and dihydrochromone to be -247.9 ( 2.3 and -204.3 ( 2.4 kJ · mol-1, respectively. Since the enthalpy of formation of hydrogen is defined as 0.0 kJ · mol-1, we thus find the enthalpies of hydrogenation to be -78.4 ( 3.5 and -84.5 ( 4.4 kJ · mol-1, respectively. By contrast, the likewise derived hydrogenation enthalpy of the aromatic naphthalene to 1,2-dihydronaphthalene (using the enthalpy of formation of the former from ref 47 and the latter from ref 48) is the far smaller -24.3 ( 1.6 kJ · mol-1. The coumarin and chromone values are comparable to that of the 1,2-dihydronaphthalene with its nonaromatic ring to tetralin (1,2,3,4-tetrahydronaphthalene; data from ref 49), numerically -100.3 ( 2.2 kJ · mol-1. Equivalently, the value for coumarin resembles that of the nonaromatic dihydronaphthalene more than that of the aromatic naphthalene, where we remind the reader that we are referring to the second “non-benzenoid” ring in the dihydrospecies. Alternatively, with precedent to the energetics of 1-ring species and the “experimental realized Dewar-Breslow model” of ref 50, we find, for the “aromatic” naphthalene and ring-opened trans-stilbene, the enthalpy of formation difference of 75.5 kJ · mol-1 while, for the “non-aromatic 1,2-dihydronaphthalene, the difference is 23.6 kJ · mol-1. The difference for coumarin and phenyl benzoate is but 20.8 kJ · mol-1. This again documents that coumarin lacks aromaticity other than found in its benzene ring. The above analysis cannot be applied to chromone. However, in that chromone contains the same groups as coumarin, the fact that coumarin is rather much more stable than chormone
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