Article pubs.acs.org/JPCA
Group Additivity Values for Estimating the Enthalpy of Formation of Organic Compounds: An Update and Reappraisal. 2. C, H, N, O, S, and Halogens John L. Holmes† and Christiane Aubry* Chemistry Department, University of Ottawa, Ottawa, Ontario, K1N 6N5 Canada S Supporting Information *
ABSTRACT: This study extends a previous publication on group additivity values (GAVs) for the elements C, H, and O, to include the elements nitrogen, sulfur, and the halogens. The present state and utility of the Benson additivity schemes for estimating the enthalpy of formation (ΔfH0) of organic compounds are again described, extending them to include more elements. Old and new GAVs for a wide variety of compounds are provided and are revised where necessary. When new terms are proposed, or old ones are significantly altered, the rationale for so doing is presented. GAV derived ring strain values for benzene and pyridine indicate that the aromatic stabilization of each is essentially the same. As before, the thermochemical consequences of replacing one functional group by another are also shown, thus permitting quick shortcuts to the estimation of new ΔfH0 values.
1. INTRODUCTION The first part of this study1 examined the additive nature of the enthalpy of formation (ΔfH0) of organic compounds containing the elements C, H, and O. Each element, or small group of elements and the atoms to which they were connected, were assigned a group additivity value (GAV), using the format developed by Benson.2 Appropriate summation of these GAVs resulted in the standard enthalpy of formation (ΔfH0) of the molecule, usually to within ±4 kJ/mol. For molecules that are sterically hindered in some way, e.g., ring strain, bulky group interactions, internal H-bond, etc., appropriate correction terms were described. For these organic compounds the additivity principle can be used with confidence to estimate the ΔfH0 for a very wide range of structures for which there are no data from either experiment or computation. The success with C, H, and O containing organic molecules was a strong inducement to extend the work to other elements. The additivity method can and certainly should be used to assess the reliability of data in collections such as the NIST WebBook,3 wherein all such data are for 298 K enthalpies of formation, as are the GAV terms in this work. Unlike the C, H, and O containing species, the data available for molecules containing C, H, O, N, S, and the halogens are relatively limited and not always reliable. Efforts have been made to select those values that yield GAV terms that are most consistent with additivity and with chemical knowledge. Nevertheless, the resulting GAV derived ΔfH0 values may sometimes poorly reproduce the available enthalpy data from experiment or computation. This is in marked contrast to the oxy analogues,1 for which generalizations concerning substitution effects proved to be strongly consistent, and exceptions were easily reconciled with steric or other significant © 2012 American Chemical Society
structural effects or, occasionally, with clearly unreliable or inadequate experimental data. For the molecules containing C, H, O, N, S, and halogens, inconsistencies will be discussed in light of the available data. Note that the GAVs presented in the tables that follow reflect all the recent experimental and computational data.
2. CARBON, HYDROGEN, AND NITROGEN Group additivity values (GAVs) for combinations of these three elements are given in Table 1, and the revised and new terms are explained in the text that follows. Note that the symbol NI− (CB)2 refers to the N atom in pyridine, CN− is the cyano group, and NC− is the isocyano group. The other terms have been identified in part 1 of this series.1 2a. Amines. Some of the GAV terms for the simple amines are little changed from the earlier Benson2 (1976) values, but they must have been established from rather few data. It should however be pointed out that the currently presented NIST3 ΔfH0 values for methylamine, ethylamine, and trimethylamine are confusing, because unsatisfactory values from long ago (1907) are also listed, requiring WebBook users to decide for themselves as to the “best” value. The ΔfH0 data for the simple aliphatic amines cited above have already been considered by the present authors in a short review article,4 and analysis of the available data allowed us to conclude that the ΔfH0 values for the first three members of this homologous series were respectively −22, −50, and −71 Received: April 19, 2012 Revised: May 28, 2012 Published: May 31, 2012 7196
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kJ/mol.4 The computations by Bond5 at the G3 and G3MP2 levels of theory also agree closely with our conclusions. For aromatic amines we have chosen CB−(N) = 25 kJ/mol, and from ΔfH0(aniline) = 83 ± 4 kJ/mol3 we derive N− (H)2(CB) = −11 ± 4 kJ/mol. The averaged data for N-methyland N,N-dimethylaniline from NIST are 84 ± 1 and 92 ± 10 kJ/mol, respectively.3 By analogy with the aliphatic amines, the first methyl substitution at nitrogen is expected to be weakly destabilizing (by ca. +3 kJ/mol) whereas the second may be weakly stabilizing, by ca. −4 kJ/mol. Thus the averaged value for the N,N-dimethyl species (92 ± 10 kJ/mol3) appears to be considerably too positive, so we assign a ΔfH0 value of ca. 80 ± 4 kJ/mol to N,N-dimethylaniline. The experimental results of Verevkin6 for benzylamine (86 ± 2 kJ/mol), α-methylbenzylamine (55 ± 1 kJ/mol), N,Ndimethylbenzylamine (81 ± 2 kJ/mol), and αα-dimethylbenzylamine (22 ± 1.5 kJ/mol) allow useful new GAV terms to be evaluated. An alternative approach can be used to estimate the enthalpies of formation of 1- and 2-naphthylamines. The ΔfH0 difference between phenol and aniline is 179 ± 4 kJ/mol, from −96 to 83 ± 4 kJ/mol. This difference is similar to that reported for a wide variety of aliphatic molecules in which −OH is replaced by −NH2, and the average ΔΔfH0 = 186 ± 10 kJ/mol is close to the aromatic result. Table 2 illustrates the general result of substituting −OH by −NH2 at a variety of sites, and the result, 186 ± 10 kJ/mol, covers a wide range of structures and provides a useful shortcut for estimating ΔfH0 values. The measured enthalpies of formation of the 1- and 2naphthols are respectively −29 ± 1 and −30 ± 1 kJ/mol,3 and the above substitution produces an estimated ΔfH0 for the two naphthylamines of 149 ± 5 kJ/mol. Note that NIST3 lists ΔfH0 for these naphthylamines as 156 ± 7 and 136 ± 12 kJ/mol, for the 1- and 2-compounds respectively, implying that these two isomers do not have closely similar enthalpies of formation. The absence of any obvious steric effect that could justify the two isomers’ having distinct ΔfH0 values leads us to propose that these enthalpies should indeed be replaced by a common ΔfH0 of −149 ± 5 kJ/mol. See also the 1- and 2halonaphthalenes in section 5c. 2b. Remarks on Disubstitution in Aromatics. It is worth briefly noting here that disubstitution with many small groups in benzenes results in no significant ortho, meta, or para effect
Table 1. Group Additivity Values for Molecules Containing Carbon, Hydrogen, Nitrogen, and Oxygen term
GAV (kJ/mol)
C−(H)3(N) C−(H)2(C)(N) C−(H)(C)2(N) C−(C)3(N) C−(H)2(CB)(N) C−(H)(C)(CB)(N) C−(C)2(CB)(N) Cd−(H)(N)
−42a −23 ± 1 −18 ± 2 −13 ± 2 −26 ± 2 −15 ± 2 −6 ± 2 36b
Cd−(C)(N) CB−(N) CB−(NI)(H) CB−(NI)(CB) CB−(NI)(C) CB−(NI)(O) CB−(NI)(N) CB−(NI)(CO) CdN−(H)2 CdN−(C)2 CdN−(H)(C) CdN−(H)(Cd)
41 ± 25 10 ± 15 12 −18 1 7 28 22 23 ± 20 ±
C−(H)2(N)(CO) C−(H)(C)(N)(CO) C−(H)2(O)(CN) C−(H)2(C)(CN) C−(H)2(Cd)(CN) CB−(CN) Ct−(CN) CN−(C) CN−(Cd) CN−(CB) CN−(Ct) Cd−(Cd)(CN) C−(C)(H)2(NO2) C−(C)2(H)(NO2) CB−NO2d
−21 ± 2 −12 ± 2 105 ± 1 −21 −23c 35 ± 4 123 ± 4 115 ± 2 115 ± 2 115 ± 4 115 ± 4 166 ± 5c −25 ± 3 −25 ± 5 20 ± 2
4 2
2 4
term
GAV (kJ/mol)
N−(H)2(C) N−(H)(C)2 N−(C)3 N−(H)2(CB)
18 ± 1 65 ± 2 102 ± 2 −11 ± 4
N−(H)2(Cd) N−(H)(Cd)2 N−(H)(Cd)(C) N−(CO)(H)2 N−(CO)(C)(H) N−(CO)(C)2 NI−(H) NI−(CB)2 NI−(C) NI−(Cd)
0 ± 10 19 ± 4 41 −60 −20 17 64 ± 2 79 ± 2 96 ± 2 98 ± 2
O−(NI) CO−(H)(N) CO−(C)(N) CO−(CB)(N)
−12 ± 2 −128 −136 −128
NC−(C)
210 ± 10
ONO−(C) NO2−(C) NO2−(CB)
−25 ± 4 −35 ± 3 −25 ± 2
Assigned, as is C−(H)3(C). bAssigned Cd−(H)(C) ≡ Cd−(H)(O) ≡ Cd−(H)(N). cSee discussion below Table 13, section 4d. dFor multiple aromatic substitutions, see section 2b. a
Table 2. Effect on ΔfH0 of Replacing an −OH Group by Amino, −NH2a molecule
ΔfH0 (kJ/mol)
amine
ΔfH0 (kJ/mol)
ΔΔfH0 b (kJ/mol)
CH3OH CH3CH2OH CH3(CH2)2OH n-C4H9−OH (CH3)2CHOH (CH3)3COH c-C6H11−OH C6H5OH CH3C(O)OH C6H5C(O)OH HC(O)OH
−202 −234 −256 −275 −272 −313 −288 −96.4 ± 1 −433 −291 −379
CH3NH2 CH3CH2NH2 CH3(CH2)2NH2 n-C4H9−NH2 (CH3)2CHNH2 (CH3)3CNH2 c-C6H11−NH2 C6H5NH2 CH3C(O)NH2 C6H5C(O)NH2 HC(O)NH2
−23.5 −49 −71 −93 −84 −121 −99; (105) 87 ± 1 −238 −101 −188
179 185 185 182 188 192 189; (183) 183 195 190 191
a ΔfH0 for benzoic acid poses a problem in that there is no NIST datum. Pedley8 gives −294 ± 2 kJ/mol. The methyl ester is poorly evaluated, with NIST data of −269 (recent) and −300 (old). If the former is good, then ΔfH0 for the acid should be ca. −291 kJ/mol, using the change in ΔfH0 resulting from methyl esterification,1 and giving the (better) result, shown in the table. bAverage ΔΔfH0 is 186 ± 10 kJ/mol.
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on the ΔfH0 values. For the dimethyl-, dihydroxy-, and methylhydroxybenzenes (cresols), the NIST3 data indicate that they can be assigned common ΔfH0 values of 18 ± 2, −272 ± 4, and −129 ± 4 kJ/mol, respectively. (Note that the cis effect for the simple olefin but-2-ene is small, at 4 ± 2 kJ/mol.) Using the appropriate GAVs, the three diaminobenzenes, for which there are no available gas phase data, can confidently be assigned a common ΔfH0 of 83 ± 8 kJ/mol. The results of Emel’yanko et al.7 give the m- and p-aminotoluenes the same ΔfH0, 57 ± 1 kJ/mol, with the o-aminotoluene at 53 ± 1 kJ/ mol. A small negative ortho effect is difficult to rationalize, given the functional groups involved. Note that the effect of amino replacing −OH in the three cresols (ΔfH0 = −129 ± 5 kJ/ mol3), and using ΔΔfH0 = 186 ± 10 kJ/mol, results in ΔfH0(aminotoluenes) of 57 ± 15 kJ/mol, consistent with the above. It should however be noted that multiple substitution in aromatics can result in large discrepancies from additivity based estimates. Particularly striking are the nitrotoluenes; for example 2,4,6-trinitrotoluene has ΔfH0 of 24 ± 4 kJ/mol whereas additivity (GAV terms based on nitrobenzene itself, ΔfH0 = 67 kJ/mol3) gives −39 kJ/mol. This difficulty is also discussed in the effects of halogen substitution, in section 5c. 2c. Amides. We analyze here some GAV terms for simple amides. The NIST3 experimental data alone do not lead to a consistent set of GAVs, so it is appropriate to include recent computed ΔfH0 values. The GAVs for the amides are not greatly different from those provided by Benson, but the new data provide confirmation. The singular effect of substituting a methyl group by phenyl in a variety of carbonyl compounds allows one to validate the ΔfH0 for benzamide. ΔΔfH0 for the pairs acetic acid−benzoic acid, acetone−acetophenone, methyl ethyl ketone−phenyl ethyl ketone, and methyl propyl ketone−phenyl propyl ketone (using the data from NIST3) are remarkably constant, encompassing the small range of 133 ± 3 kJ/mol. The same substitution in acetamide, ΔfH0 = −237 ± 2 kJ/mol, gives ΔfH0(benzamide) = −104 ± 5 kJ/mol, in close agreement with the value shown in Table 3. The experimental results for methyl substitution at N in amides are similar to those for amines but apparently are somewhat more negative; e.g., the effect of one such substitution is small but negative, −4 kJ/mol for formamide and −7 for acetamide, whereas for amines it is positive (ca. 3 kJ/mol), and for two methyl substitutions it is also negative, −7 kJ/mol for formamide and −15 kJ/mol for benzamide (−4 kJ/ mol for an amine), but +7 kJ/mol for propanamide. Whether the magnitude of these differences is significant is uncertain. In marked contrast, methyl substitution at −OH attached to −C, −CO, or −Cd results in a constant positive change of 15−17 kJ/ mol. 2d. Nitriles and Isonitriles. The GAV terms for the nitrile function are based on the use of the common terms for single (C), double (Cd), and triple (Ct) bonded carbon atoms, a new aromatic CB−(CN), and new GAV terms for the attached −CN group. As can be seen in Table 1 these last are almost constant. (Note that Benson2 combined these terms for Cx−(CN) or −(NC), into single GAVs.) Thermochemical data for isonitriles in NIST3 are few, but some new data have recently appeared. ΔfH0 values for CH3NC by computation and by experiment10 are 174.5 ± 1.5 and 173 ± 0.4 kJ/mol, respectively; also reported were ΔfH0 values for the
Table 3. ΔfH0 values for some simple amides molecule
ΔfH0 a (kJ/mol)
GAV ΔfH0 (kJ/mol)
HC(O)NH2 HC(O)NHCH3 HC(O)N(CH3)2 CH3C(O)NH2 CH3C(O)NHCH3 CH3C(O)N(CH2CH3)2 CH3CH2C(O)NH2 CH3CH2C(O)N(CH3)2 CH3CH2CH2C(O)NH2 CH3(CH2)3CONH2 (CH3)2CHC(O)NH2 (CH3)3CC(O)NH2 C6H5C(O)NH2 C6H5C(O)N(CH3)2
−186, −190 −1928 −195b −238, −2365 −248 ± 6 −287 −259, −259,5 −2555 −250 −279, −279,5 −2755 −2989 −283, −286,5 −2815 −313 −101 −86,1 −83 ± 9
−188 −185 −192 −238 −235 −296 −260 −264c −281 −302 −284 −313 −101d −108c
5
NIST or otherwise stated. All experimental values are ±4 kJ/mol or less, except where noted. Data derived from computational chemistry are in italics. bSee discussion. cNote that these results depend upon the assumption that the sequence N−(CO)(H)2 → N−(CO)(H)(C) → N−(CO)(C)2 (−60 → −20 → 17 kJ/mol) is similar to that for N− (H)2(C) → N−(H)(C)2 → N−(C)3, where the differences between the latter are 45 and 35 kJ/mol, respectively. dNote that this result depends upon the CO−(CB)(N) term derived from this molecule and inter alia shows that the effect of replacing −OH by −NH2 at an aromatic site is slightly (ca. −6 kJ/mol) less positive than for alkyl compounds. (Similar effects are found for replacing H− and CH3− by −NH2 in aldehydes and ketones.) a
ethyl (145 ± 8, 143 ± 1 kJ/mol), isopropyl (117 ± 8, 107.5 ± 1 kJ/mol) and tert-butyl analogues (88 ± 4, 76 ± 1 kJ/mol). The derived additivity term for the isonitrile GAV, NC−(C), using the hydrocarbon terms for C−(H)3, C−(C)(H)2, C− (C)2(H), and C−(C)3, ranges from 203 to 216, but without any clear trend among the data, and therefore the NC−(C) GAV is assigned the average result of 210 ± 8 kJ/mol. 2e. Aminoalkenes: Substitutions at C and N. Some of the available data for the substituted aminoalkenes present an inconsistent picture. A value for the simple term N−(Cd)(H)2 can be estimated from ΔfH0(CH2CHNH2) for which there are some data, namely ΔfH0(ion) = 826 ± 8 kJ/mol (experiment)11 and 849 and 846 kJ/mol (G2(MP2) and CBS-Q, respectively).12 NIST3 lists the adiabatic ionization energy as 8.10 eV, leading to ΔfH0(CH2CHNH2) = 44 ± 8, 67, and 64 kJ/mol, respectively. Neglecting the value derived from the experimental cationic enthalpy of formation, an averaged ΔfH0 value for (CH2CHNH2) of 62 ± 5 kJ/mol is obtained. This ΔfH0 can also be estimated by consideration of substitution effects, in this case, replacing a functionality by the −NH2 group. In section 2a, this effect is shown to be 186 ± 10 kJ/mol for replacing −OH by −NH2 in aromatic and aliphatic compounds. For vinylamine therefore, going from the alcohol, CH2CHOH (ΔfH0 = −126 kJ/mol),3 to CH2CHNH2 results in ΔfH0 = 59 ± 10 kJ/mol, close to the above computational data. Using an averaged ΔfH0(CH2CHNH2) = 62 ± 10 kJ/mol, combined with Cd−(H)2 = 26 kJ/mol and choosing Cd− (H)(N) ≡ Cd−(H)(O) = 36 kJ/mol, results in the GAV term N−(Cd)(H)2 = 0 ± 10 kJ/mol. Computed enthalpy of formation values are also available for alkyl substituted vinylamines, CH3CHCHNH2, (CH3)2C CHNH2, and CH3CH2CHCHNH2 (Table 4). Note that the effect of substituting alkene hydrogen by methyl in vinylamine is quite close to that observed for the simple alkene pair 7198
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Table 4. Effect of Methyl Substitution on the −CC−N− Function ΔfH0 a (kJ/mol)
molecule
67, 64, 59 ± 2, 62 ± 5c 36, 38, 3014 35 ± 4c 6, 7 6 ± 1c 16, 18 17 ± 1c
CH2CHNH2 CH3CHCHNH2 (CH3)2CCHNH2 CH3CH2CHCHNH2
13
ΔΔfH0 c
molecule
60, 62
ΔfH0 b (kJ/mol)
ΔΔfH0 c
CH2CH2
52
−27d
CH3CHCH2
20
−32d
−29e
(CH3)2CCH2
−18
−38e
−18d
CH3CH2CHCH2
−0.63
−18d
Values are from ref 12 unless otherwise stated. bValues are from NIST.3 cAverage value used in calculation. dΔΔfH0 is calculated with respect to unmethylated species. eΔΔfH0is calculated with respect to methylated species.
a
Table 5. ΔfH0 Data for Substituted Pyridines molecule
ΔfH0 a (kJ/mol)
GAVb (kJ/mol)
2-methyl 3-methyl 4-methyl
100 ± 2 109 ± 5 103 ± 2
100 108 108
2-acetyl 3-acetyl 4-acetyl
−41 ± 318 −35 ± 318 −36 ± 218
−35 ± 3 −28 ± 4 −28 ± 4
2-hydroxy 3-hydroxy 4-hydroxy
−80 ± 213,c −44 ± 213 −41 ± 213
2-amino 3-amino 4-amino
118 ± 1 144 ± 2 130 ± 1
−38 ± 5 −38 ± 5
141 141
ΔfH0 a (kJ/mol)
molecule
GAVb (kJ/mol)
2,3-dimethyl 2,4-dimethyl 2,5-dimethyl 2,6-dimethyl 3,4-dimethyl 3,5-dimethyl
68 64 66 57 ± 3 70 73
68 68 68 61 74 74
2-methyl-3-hydroxy 2-methyl-4-hydroxy 2-methyl-5-hydroxy 2-methyl-6-hydroxyc 2-carboxylic acid19 3-carboxylic acid19 4-carboxylic acid19
−84 ± 2 −71 ± 2 −70 ± 3 −120 ± 2c −243 ± 3 −222 ± 2 −235 ± 5
−78 ± 7 −78 ± 7 −78 ± 7
2-carboxylic acid methyl ester19 3-carboxylic acid methyl ester19 4-carboxylic acid methyl ester19
−209 ± 2 −214 ± 2 −219 ± 2
−228 −219 −219
−249 −241 −241
a Averaged values from ref 3. bUsing the new GAV terms described below. All ±4 kJ/mol except as stated otherwise. cThese unexpected experimental values should be attributed to 2-pyridones; see discussion below.
CH2CH2 and CH3CHCH2 (−32 kJ/mol for the alkene vs −27 kJ/mol for the N-compound). When a second methyl is substituted, the effect on ΔfH0 is to reduce it somewhat further, being −29 kJ/mol for the aminoalkene compared to −38 kJ/ mol for the alkene. For the hydroxy analogues, methyl substitution in vinyl alcohol results in changes of −31 kJ/mol for the first CH3− and−36 kJ/mol for the second. That these enthalpy changes differ so much between these two structurally similar molecules suggests that some of the data may merit reevaluation. Using N−(Cd)(H)2 = 0 ± 10 kJ/mol combined with the computed enthalpies of formation12 of CH2C(CH3)NH2 (21 and 24 kJ/mol) and CH3CHC(CH3)NH2 (−2 and −4 kJ/ mol) and for CH2C(CH2CH3)NH2 (4 and 6 kJ/mol) leads to Cd−(C)(N) = 41 ± 4 kJ/mol. This results in the GAV sequence Cd−(H)2 → Cd−(H)(N) → Cd−(C)(N) (26, 36, and 41 kJ/mol) similar to and consistent with that for the carbon and oxygen analogues, which are 26, 36, and 43 kJ/ mol.1 2f. Pyrrole. To evaluate ΔfH0 for pyrrole by GAVs, we need a new term, N−(H)(Cd)2, that can be derived from ΔfH0 for divinylamine. A computed value of ΔfH0((CH2CH)2NH) = 122 kJ/mol has been published,15 and using the established GAV terms for Cd−(H)2 and Cd−(N)(H) results in N− (H)(Cd)2 = −2 ± 4 kJ/mol. This GAV yields ΔfH0 pyrrole =
122 kJ/mol without any correction for ring strain or stabilization. It is however to be expected that some quasiaromatic stabilization, clearly observed in furan and thiophene (see below), −251 and −57 kJ/mol, respectively, will be repeated here. The reference values for pyrrole in NIST3 are in very poor agreement: 143 and 108 kJ/mol. The latter is that selected by Pedley,8 and a more recent computed value of 109 kJ/mol has been reported.16 The above GAV result for pyrrole, 122 kJ/ mol, is therefore incompatible with both reference values, leading to either a ring strain of +21 or a stabilization of ca. −14 kJ/mol. We will argue that the above computed value for ΔfH0(divinylamine) is incorrect. The effect of vinyl substitutions at O and at N should be similar. Thus the series methanol to methylvinyl ether to divinyl ether results in ΔfH0 values of −202, −105 ± 3, and 13 ± 4 kJ/mol, respectively. For the first two N-analogues, starting with methylamine, ΔfH0 = −22 kJ/mol, and vinylamine, ΔfH0 = 61 kJ/mol, using a new computed value for ΔfH0(divinylamine), 143 kJ/mol (G3(MP2)B3),17 shows a similar trend and results in GAV(N−(H)(Cd)2) = 19 ± 4 kJ/ mol. With this latter value and using the GAV terms for Cd− (H)(N) and Cd−(H)(Cd), ΔfH0(pyrrole) is calculated to be 148 ± 4 kJ/mol without taking into account the effect of any 7199
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the ΔfH0 value for the 2-amino derivative, 118 kJ/mol, leads to 1 ± 4 kJ/mol. A few ΔfH0 values for mixed substituted pyridines are also in the NIST database.3 They are the 3-, 4-, and 5-hydroxy-(2methylpyridines), with ΔfH0 values of −84.5 ± 1.8, −71.7 ± 1.7, and −69.8 ± 2.6 kJ/mol, respectively, indicating a possible secondary positional effect. Note the unexpectedly lower value for the 3-hydroxy species. There are also data for the o-, m-, and p-aminophenols,3 but the ΔfH0 values from two separate data sets are in poor agreement. For the ortho-substituted phenols the ΔfH0 values are −104.4 ± 1.7 and −87.1 ± 1.3 kJ/mol, for the metasubstituted species the ΔfH0 values are −98.6 ± 1.6 and −89.4 ± 1.6 kJ/mol, and finally for the para-substituted compound the ΔfH0 values are −90.5 ± 1.2 and −81.5 ± 1.7 kJ/mol. Although the averaged ΔfH0 values are quite close, −96 ± 3, −94 ± 4, and −86 ± 4 kJ/mol, respectively, no simple parallel can be drawn with the pyridine analogues. No other ΔfH0 data for simple substituted pyridines could be found. The ΔfH0 of pyridine N-oxide has been determined by experiment and by computation,20 and the average result is 137 ± 4 kJ/mol. The GAV for O−(NI) is therefore small, at −3 ± 7 kJ/mol. Substituted pyridine N-oxides have also been reported by experiment21 and by computation.20 However, the agreement between the data is poor; for example, for the 4-methyl derivative ΔfH0 is reported as 91 ± 321 and 10520 kJ/mol. However, the effect of the N-oxygen atom appears again to be small: ΔfH0 for 3-methylpyridine is 109 ± 5 kJ/mol (see Table 5). ΔfH0 for the three pyridinecarboxylic acids and their methyl esters19 derived from experiment have been reported and are shown in Table 5. The data are somewhat scattered, but the 2derivatives have ΔfH0 values slightly but significantly below their 3- and 4-isomers by margins similar to those for the methyl- and acetylpyridines. The average ΔΔfH0 between the acids and their methyl esters is 18 kJ/mol, close to that typical of such a substitution, 21 ± 2 kJ/mol.1 2h. GAV Terms for Double-Bonded N, Imino Nitrogen, NI. For vinylimine, the NIST source3 gives three values: ΔfH0(HNCH2) = 69 ± 8, 110 ± 8 (the original paper lists 105 kJ/mol14), and 110 ± 10 kJ/mol. More recent values give a smaller range, starting with an estimate of
