Enthalpies of combustion of the pyridine N-oxide derivatives: 4-methyl-, 3-cyano-, 4-cyano-, 3-hydroxy-, 2-carboxy-, 4-carboxy-, and 3-methyl-4-nitro, and of the pyridine derivatives: 2-carboxy-, and 4-carboxy-. The dissociation enthalpies of the N-O bonds

Article abstract of DOI:10.1006/jcht.1998.0353

The standard (p^o = 0.1 MPa) molar enthalpies of formation ΔfH^o_m(cr) at T = 298.15 K were determined using static-bomb calorimetry for crystalline 4-methylpyridine N-oxide (4MePyNO), 3-cyanopyridine N-oxide (3CNPyNO), 4-cyanopyridine N-oxide (4CNPyNO), 3-hydroxypyridine N-oxide (3OHPyNO), 2-pyridinecarboxylic acid N-oxide (2CO₂HPyNO), 4-pyridinecarboxylic acid N-oxide (4CO₂HPyNO), 3-methyl-4-nitropyridine N-oxide (₃Me4NO₂PyNO), 2-pyridinecarboxylic acid (2CO₂HPy), and 4-pyridinecarboxylic acid (4CO₂HPy). The standard molar enthalpies of sublimation Δ^g_crH^o_m at T = 298.15 K were measured by microcalorimetry, or by a mass-loss effusion technique, and from the enthalpies of formation of the gaseous compounds the dissociation enthalpies D^o_m of the (N⁺-O^-) dative covalent bonds were derived. Δ_fH^o_m(cr)/(kJ · mol^-1) Δ^g_crH^o_m/(kJ · mol^-1) D^o_m(N-O)/(kJ · mol^-1) 4-MePyNO 5.6 ± 2.1 85.3 ± 2.6 262.4 ± 3.4 3-CNPyNO 170.9 ± 1.4 101.9 ± 2.0 254.3 ± 3.1 4-CNPyNO 162.8 ± 1.4 104.4 ± 4.3 265.5 ± 4.6 3-OHPyNO -171.3 ± 1.0 121.8 ± 4.4 255.0 ± 4.8 2-CO₂HPyNO -364.1 ± 1.8 94.4 ± 4.0 275.9 ± 5.1 4-CO₂HPyNO -381.2 ± 1.3 136.1 ± 1.2 259.5 ± 5.0 3-Me4NO₂PyNO -19.5 ± 3.4 106.7 ± 2.0 263.4 ± 7.1 2-CO₂HPy -341.0 ± 1.2 98.0 ± 2.3 4-CO₂HPy -348.7 ± 1.6 113.9 ± 4.4 Comparison has been made with D^o_m(N-O) values in pyridine N-oxide derivatives.

Full text of DOI:10.1006/jcht.1998.0353

J. Chem. Thermodynamics 1998, 30, 869᎐878
Article No. ct980353
Enthalpies of combustion of the pyridine N-oxide
derivatives: 4-methyl-, 3-cyano-, 4-cyano-,
3
3
2
-hydroxy-, 2-carboxy-, 4-carboxy-, and
-methyl-4-nitro, and of the pyridine derivatives:
-carboxy-, and 4-carboxy-. The dissociation
enthalpies of the N-O bonds
Maria D. M. C. Ribeiro da Silva, M. Agostinha, R. Matos,
M. Cl ´a udia Vaz, Lu ´ı s M. N. B. F. Santos,
Centro de In¨estiga ¸c a˜ o em Qu ´ı mica, Departamento de Qu ´ı mica, Faculdade
de Ci ˆe ncias, Uni¨ersidade do Porto, Rua do Campo Alegre, 687,
P-4150 Porto, Portugal
G. Pilcher,
Department of Chemistry, Uni¨ersity of Manchester, Manchester M13 9PL, U.K.
W. E. Acree Jr., and Joyce R. Powell
Department of Chemistry, Uni¨ersity of North Texas, Denton, TX 76203-5070,
U.S.A.
T
The standard Ž pЊ s 0.1 MPa. molar enthalpies of formation ⌬ H Žcr. at T s 298.15 K were
f
m
determined using static-bomb calorimetry for crystalline 4-methylpyridine N-oxide
Ž4MePyNO., 3-cyanopyridine N-oxide Ž3CNPyNO., 4-cyanopyridine N-oxide Ž4CNPyNO.,
3
4
-hydroxypyridine N-oxide Ž3OHPyNO., 2-pyridinecarboxylic acid N-oxide Ž2CO HPyNO.,
2
-pyridinecarboxylic acid N-oxide Ž4CO HPyNO., 3-methyl-4-nitropyridine N-oxide
2
Ž3Me4NO PyNO., 2-pyridinecarboxylic acid Ž2CO HPy., and 4-pyridinecarboxylic acid
2
2
g
cr
T
Ž4CO HPy.. The standard molar enthalpies of sublimation ⌬ Hm at T s 298.15 K were
2
measured by microcalorimetry, or by a mass-loss effusion technique, and from the enthalpies
T
m
q
y
of formation of the gaseous compounds the dissociation enthalpies D of the ŽN -O
.
dative covalent bonds were derived.
T
m
Žcr.rŽkJ moly1.
.
g
cr
T
m
rŽkJ moly1. D ŽN-O.rŽkJ moly1.
.
T
m
.
⌬f
H
⌬
H
4
3
4
3
2
4
3
2
4
-MePyNO
-CNPyNO
-CNPyNO
-OHPyNO
-CO 2HPyNO
5.6 " 2.1
170.9 " 1.4
162.8 " 1.4
y171.3 " 1.0
y364.1 " 1.8
y381.2 " 1.3
y19.5 " 3.4
y341.0 " 1.2
y348.7 " 1.6
85.3 " 2.6
262.4 " 3.4
254.3 " 3.1
265.5 " 4.6
255.0 " 4.8
275.9 " 5.1
259.5 " 5.0
263.4 " 7.1
101.9 " 2.0
104.4 " 4.3
121.8 " 4.4
94.4 " 4.0
136.1 " 1.2
106.7 " 2.0
98.0 " 2.3
113.9 " 4.4
-CO HPyNO
2
-Me4NO PyNO
2
-CO HPy
2
-CO HPy
2
T
m
Comparison has been made with D ŽN-O. values in pyridine N-oxide derivatives.
ᮊ 1998
Academic Press
KEYWORDS: pyridine N-oxide derivatives; enthalpy of combustion; enthalpy of sublimation;
energetics of N-oxide derivatives
0021᎐9614r98r070869 q 10 $30.00r0
ᮊ 1998 Academic Press

8
70
M. D. M. C. Ribeiro da Silva et al.
1
. Introduction
q
y
The dissociation enthalpy of the ŽN -O . dative covalent bond has been reported
T
m
y1
for 4-nitropyridine N-oxide as D ŽN-O.rŽkJ.mol . s Ž258.5 " 5.3., and for 3-
T
y1
Ž1.
pyridinecarboxylic acid N-oxide as D ŽN-O.rŽkJ.mol . s Ž252.8 " 2.7.. The
m
q
y
dissociation enthalpy of the ŽN -O . dative covalent bond shows variation
depending on the molecular environment of the bond. The effect of substitution of
T
m
various groups into the pyridinic ring of pyridine N-oxides upon D ŽN-O. has been
investigated here by measuring the molar enthalpies of combustion and of
sublimation of the crystalline compounds 4-methylpyridine N-oxide, 3-, and 4-
cyanopyridine N-oxide, 3-hydroxypyridine N-oxide, 2-, and 4-pyridinecarboxylic acid
N-oxide, 3-methyl-4-nitropyridine N-oxide, and 2-, and 4-pyridinecarboxylic acid.
2
. Experimental
2
-Pyridinecarboxylic acid Ž2CO HPy., 4-pyridinecarboxylic acid Ž4CO HPy., 2-
2 2
pyridinecarboxylic acid N-oxide Ž2CO HPyNO., and 4-cyanopyridine N-oxide
2
Ž4CNPyNO. obtained from Aldrich Chem. Co., and 3-hydroxypyridine N-oxide
Ž3OHPyNO., and 4-pyridinecarboxylic acid N-oxide Ž4CO HPyNO. obtained from
2
Acros Chem. Co. were each purified by four crystallizations from methanol.
4
-Methylpyridine N-oxide Ž4MePyNO. and 3-cyanopyridine N-oxide Ž3CNPyNO.
were prepared by oxidation of 4-methylpyridine and 3-cyanopyridine, respectively,
using hydrogen peroxide in ethanoic acid in accordance with published procedures
for preparing pyridine N-oxide derivatives.Ž2 ᎐ 4. The pyridine derivatives were
dissolved in Žethanoic acid q benzene. to which was added 35 per cent hydrogen
peroxide and the resulting solution was heated for several hours at T s 323 K. The
solutions were then cooled, concentrated under reduced pressure, and neutralized
with aqueous ammonia. The crude pyridine N-oxides were extracted with
trichloromethane and then purified by four crystallizations from benzene
Ž4MePyNO. and methanol Ž3CNPyNO.. 3-Methyl-4-nitropyridine N-oxide
Ž3Me4NO PyNO. was prepared by nitrating 3-methylpyridine N-oxide with Žfuming
2
Ž5.
nitric acid q concentrated sulfuric acid. as described by Badger and Rao, and
the product was purified by four crystallizations from methanol. Analysis by t.l.c. on
all of the pyridine and pyrydine N-oxide derivatives showed just one spot.
Elemental analyses were in agreement with expected values and the compositions
in mass fractions were: for 3HOPyNO, C H NO ; found: C, 0.5398; H, 0.0460; N,
5
5
2
0
.1265; calculated: C, 0.5405; H, 0.0454; N, 0.1261; for 2CO HPy, C H NO ;
2 6 5 2
found: C, 0.5845; H, 0.0412; N, 0.1135; calculated: C, 0.5854; H, 0.0409; N, 0.1138;
for 4CO HPy; found: C, 0.5867; H, 0.0407; N, 0.1126; calculated: C, 0.5854; H,
2
0
0
0
3
0
.0409; N, 0.1138; for 2CO HPyNO, C H NO ; found: C, 0.5187; H, 0.0365; N,
2 6 5 3
.1015; calculated: C, 0.5180; H, 0.0362; N, 0.1007; for 4CO HPyNO; found: C,
2
.5175; H, 0.0359; N, 0.0996; calculated: C, 0.5180; H, 0.0362; N, 0.1007; for
CNPyNO, C H N O; found: C, 0.6005; H, 0.0345; N, 0.2329; calculated: C,
6
4
2
.6000; H, 0.0336; N, 0.2332; for 4CNPyNO; found: C, 0.6010; H, 0.0330; N, 0.2342;
calculated: C, 0.6000; H, 0.0336; N, 0.2332; for 4MePyNO; C H NO; found: C,
6
7

T
m
D
ŽN-O. in pyridine N-oxides
871
0
3
0
.6599; H, 0.0661; N, 0.1287; calculated: C, 0.6604; H, 0.0647; N, 0.1284; and for
Me4NO PyNO, C H N O ; found: C, 0.4670; H, 0.0405; N, 0.1819; calculated: C,
.4676; H, 0.0392; N, 0.1818.
2
6
6
2
3
The bomb calorimeter, the subsidiary apparatus, and the operating technique
Ž6,7.
have been reported elsewhere.
The energy equivalent of the calorimeter was
determined from the combustion of benzoic acid ŽBureau of Analysed Samples
CRM 190-p. as having a massic energy of combustion under standard bomb
y1
conditions of yŽ26431.8 " 3.7. J .g . From nine calibration experiments
y1
Žcalor. s Ž15911.2 " 1.5. J .K
␧
for an average mass of water added to the
calorimeter of 3119.6 g; the uncertainty quoted is the standard deviation of the
mean. This value of ␧Žcalor. applies to all the measurements except those for
2
CO HPy for which some changes to the system made recalibration necessary, and
2
y1
from eight experiments ␧Žcalor. s Ž15908.3 " 1.0. J .K . Hexadecane ŽAldrich
Gold Label. was used as an auxiliary combustion aid in some measurements and its
standard massic energy of combustion was measured separately to be
y1
y⌬ uЊŽl.rŽJ .g . s Ž47156.9 " 1.2., a value in agreement with that of Fraser and
Prosen
c
Ž8.
y1
for a sample of mass fraction purity of 0.9996, y⌬ uЊŽl.rŽJ .g . s
c
Ž47155.0 " 3.8.. Samples in pellet form were ignited at T s Ž298.150 " 0.001. K in
3
oxygen at a pressure p s 3.04 MPa, with a volume of 1 cm of water added to the
bomb. The electrical energy for ignition was determined from the change in the
potential difference across a capacitor when discharged through the platinum
ignition wire. For the cotton thread fuse of empirical formula CH 1.686O0.843, the
y1 Ž9.
.
massic energy of combustion y⌬ uЊ s 16250 J g
confirmed in our laboratory. Corrections for carbon formation were based on
y33 kJ g for the massic energy of combustion of C cr . Corrections for nitric
acid formation were based on y59.7 kJ mol for the molar energy of formation of
.
This value was previously
c
y1
Ž9.
.
Ž .
y1
.
y3
Ž10.
.
Ž
.
Ž .
Ž
.
0
.1 mol dm HNO aq from N , O , and H O l . At T s 298.15 K, Ѩ urѨ p
3
2
2
2
T
y1
y1
.
.
for these solids was assumed to be y0.2 J g MPa , a value typical for organic
solids. For each compound, y⌬ uЊ was calculated by the procedure given by
c
Ž11.
Hubbard et al.
The relative atomic masses were those recommended by the
Ž12.
IUPAC Commission.
The amount of substance used in each experiment was
determined from the total mass of carbon dioxide produced after allowance for
that formed from the cotton thread fuse and hexadecane, and that lost due to
carbon formation. The average ratios of the mass of carbon dioxide produced by
the samples during the energy of combustion measurement to that calculated from
its mass with uncertainties of twice the standard deviation of the mean were: for
4
MePyNO, Ž0.9981 " 0.0006.; for 3CNPyNO, Ž0.9998 " 0.0002.; for 4CNPyNO,
Ž1.0002 " 0.0003.; for 3OHPyNO, Ž0.9998 " 0.0001.; for 2CO HPyNO, Ž0.9999 "
0
0
2
.0001.; for 4CO HPyNO, Ž1.0002 " 0.0001.; for 3Me4NO PyNO, Ž1.0000 "
2
2
.0003.; for 2CO HPy, Ž0.9999 " 0.0001.; for 4CO HPy, Ž1.0001 " 0.0002..
2
2
Except for 4CO HPy, the standard molar enthalpies of sublimation were
2
Ž13.
measured by the ‘‘vacuum sublimation’’ drop microcalorimetric method. Samples
of about 5 mg of each compound, contained in a thin glass capillary tube sealed at
one end, were dropped at room temperature into the hot reaction vessel of the
Calvet High Temperature Microcalorimeter ŽSETERAM HT 1000. held at

8
72
M. D. M. C. Ribeiro da Silva et al.
temperatures between T s 400 K and T s 500 K, depending on the compound,
and then removed from the hot zone by vacuum sublimation. The observed
T
m
T
m
standard molar enthalpies of sublimation
Ä H Žg, T. y H Žcr, 298.15 K.4
were
T
T
m
corrected to T s 298.15 K using ⌬
H Žg. estimated by a group contribution
2
98.15 K
Ž14.
method based on the values of Stull et al.
The microcalorimeter was calibrated
in situ for these measurements by making use of the reported molar enthalpy of
y1 Ž15.
sublimation of naphthalene, C H : Ž72.51 " 0.01. kJ.mol
.
1
0
8
The standard enthalpy of sublimation of 4CO HPy was measured by the Knudsen
2
Ž16.
method using the apparatus described by Burkinshaw and Mortimer,
with
Ž1.
modifications reported previously. The vapour effusing from the Knudsen cell
was allowed to condense on a quartz crystal positioned above the effusion hole and
changes in the frequency of oscillation of the quartz were proportional to the mass
condensed. From the Knudsen formula:
p s
Ž
d mrdt
.
ay1
Ž
2␲ RTrM
.
1r2,
1
Ž .
where Žd mrdt. is the rate of mass loss, a is the effective hole area, and M is the
molar mass of the effusion vapour. As the rate of change of frequency of
g
T
m
oscillation Žd␯rdt. is proportional to the rate of mass loss, ⌬ H may be derived
cr
1
r2
1r2 y2
y1
from the slope of ln
Ä
ŽT .d␯rdt.rŽK .s
.4
against T
by applying the
integrated form of the Claussius᎐Clapeyron equation. Measurements were made in
the temperature range T s 345 K to T s 392 K and the molar enthalpy of
sublimation at the mean temperature was corrected to T s 298.15 K using
g
T
y1
y1
.
.
mol , an assumed value typical for the sublimation of
⌬
C
s y50 J K
cr p, m
Ž16.
organic solids.
3
. Results
Results for a typical combustion experiment on each compound are given in table 1;
⌬mŽH O. is the deviation of the mass of water added to the calorimeter from
2
3
119.6 g, the mass assigned for ␧Žcalor.; ⌬U⌺ is the correction to the standard
Ž11.
state; and the remaining terms are as previously described.
were ignited at T s 298.15 K, ⌬UŽIBP. s y
⌬T q ⌬UŽign., where ⌬T is the calorimeter temperature change corrected
As samples
Ä
␧Žcalor. q c ŽH O, l..⌬ ŽH O. q
p
2
m
2
␧
4
f
ad
ad
for heat exchange and the work of stirring. For 2CO HPy, ␧Žcalor. s Ž15908.3 "
2
y1
y1
.0. J .K , whereas for all the other compounds ␧Žcalor. s Ž15911.2 " 1.5. J .K
The individual values of y⌬ uЊ together with the mean and its standard deviation
1
.
c
are given in table 2. Table 3 lists the derived standard molar enthalpies of
combustion and of formation in the crystalline and gaseous states. In accordance
with normal thermochemical practice, the uncertainties assigned to the standard
molar enthalpies of combustion and formation are twice the overall standard
deviation of the mean and include the uncertainties in calibration and in the
T
T
auxiliary quantities used. To derive ⌬ H from ⌬ H the standard molar
f
m
c
m
y1
enthalpies of formation for H OŽl.: yŽ285.83 " 0.04. kJ.mol
and for CO Žg.:
2
2
y1
Ž17.
yŽ393.51 " 0.13. kJ.mol , were used.

T
m
D
ŽN-O. in pyridine N-oxides
873
␧
␧

8
74
M. D. M. C. Ribeiro da Silva et al.

T
m
D
ŽN-O. in pyridine N-oxides
875
T
TABLE 3. Derived standard molar energies of combustion in the crystalline state ⌬ U Žcr., standard
c
m
T
m
molar enthalpies of combustion in the crystalline state ⌬ H Žcr., standard molar enthalpies of
sublimation ⌬ H , and standard molar enthalpies of formation in the gaseous state ⌬ H Žg., at
c
g
cr
T
m
T
m
f
T s 298.15 K Ž pЊ s 0.1 MPa.
T
m
T
m
T
m
g
T
T
m
y⌬ U Žcr.
y⌬ H Žcr.
⌬ H Žcr.
⌬
Hm
⌬ H Žg.
c
c
f
cr
f
.
y1
.
y1
.
y1
.
y1
.
kJ mol
y1
kJ mol
kJ mol
kJ mol
kJ mol
4
3
4
3
2
4
3
2
4
MePyNO
CNPyNO
CNPyNO
OHPyNO
3365.2 " 1.9
3104.8 " 1.1
3096.7 " 1.1
2511.4 " 0.8
2713.4 " 1.6
2696.3 " 1.2
3201.6 " 3.3
2735.2 " 0.9
2727.5 " 1.4
3367.1 " 1.9
3103.6 " 1.1
3095.5 " 1.1
2510.8 " 0.8
2711.5 " 1.6
2694.4 " 1.2
3199.1 " 3.3
2734.6 " 0.9
2726.9 " 1.4
5.6 " 2.1
170.9 " 1.4
162.8 " 1.4
y171.3 " 1.0
y364.1 " 1.8
y381.2 " 1.3
y19.5 " 3.4
y341.0 " 1.2
y348.7 " 1.6
85.3 " 2.6
101.9 " 2.0
104.4 " 4.3
121.8 " 4.4
94.4 " 4.0
136.1 " 1.2
106.7 " 2.0
98.0 " 2.3
113.9 " 4.4
90.9 " 3.3
272.8 " 2.4
267.2 " 4.5
y49.5 " 4.5
y269.7 " 4.4
y245.1 " 1.8
87.2 " 3.9
CO HPyNO
2
CO HPyNO
2
Me4NO PyNO
2
CO HPy
y243.0 " 2.6
2
CO HPy
y234.8 " 4.7
2
4
. Discussion
T
There are two previously reported values for ⌬ H Žcr. of 4-methylpyridine N-oxide.
f
m
From measurement of its enthalpy of reduction to 4-methylpyridine using TiŽIII.Cl ,
3
Ž18.
T
m
y1
reported ⌬ H Ž4MePyNO, cr. s Ž12.9 " 0.9. kJ.mol
Airoldi and Gon ¸c alves
,
f
Ž19.
and from measurement of the enthalpy of combustion Lebedev et al.
reported
T
m
y1
⌬f H Ž4MePyNO, cr. s yŽ0.5 " 2.1. kJ.mol , neither value is in agreement with
y1
the value reported in the present paper of Ž5.6 " 2.1. kJ.mol . The TiŽIII.Cl3
reduction method has been shown to be unreliable,Ž1. and furthermore, as this
compound is hygroscopic, the combustion result based on the mass of carbon
dioxide produced should be more reliable than that based on the mass of the
sample used. The enthalpy of combustion of 2-pyridinecarboxylic acid was
Ž20.
T
m
measured by Gubareva et al.
who reported ⌬ H Ž2CO HPy, cr. s yŽ343.8 "
f
2
y1
.8. kJ.mol , a value which agrees within the limits of uncertainty with that in the
1
y1
present paper, yŽ341.0 " 1.2. kJ.mol
.
T
m
D ŽN-O. is the enthalpy of the dissociation reaction R-c-C H NOŽg. s R-c-
5
4
T
m
y1
Ž17.
C H NŽg. q OŽg., which requires ⌬ H ŽO, g.rŽkJ.mol . s Ž249.18 " 0.10..
5
4
f
T
D ŽN-O. for the compounds studied in the present paper and values for other
pyridine N-oxide derivatives are listed in table 4. The earlier conclusion that the
m
Ž1.
molar dissociation enthalpy of the ŽN-O. bond in pyridine N-oxide derivatives
y1
should lie within the region Ž260 " 10. kJ.mol
is confirmed, except for 2-
pyridinecarboxylic acid N-oxide, but this molecule exhibits an intramolecular
Ž23.
hydrogen bond. Crystal structure determination by Laing and Nicholson
showed
the O иии O distance of the intramolecular hydrogen bond to be 239 pm, distinctly
shorter than that usually found in simple Ž-O-H иии O. bonds of 250 pm to 280 pm.
T
m
y1
D ŽN-O. in 2-pyridinecarboxylic acid N-oxide is only ca. 20 kJ.mol greater than
that in the 3-, and 4-pyridinecarboxylic acid N-oxides, a smaller increase than
would be expected for such a short hydrogen bond. It appears, however, that
2
-pyridinecarboxylic acid also possesses an intramolecular hydrogen bond in this

8
76
M. D. M. C. Ribeiro da Silva et al.
T
TABLE 4. Values of enthalpies of dissociation D ŽN-O., and standard molar enthalpies of formation
m
T
in the gaseous state ⌬ H Žg., in pyridine N-oxides
f
m
⌬f H Žg.rŽkJ .moly1.
T
m
D ŽN-O.rŽkJ .moly1.
T
m
R-c-C H NO
R-c-C H N
5 4
5
4
Ž21.
Ž18.
Ž18.
Ž22.
Ž22.
Ž22.
Ž22.
Ž22.
c-C H NO
124.7 " 1.9
88.5 " 2.3
91.0 " 2.5
140.4 " 0.7
99.2 " 0.8
106.4 " 0.6
104.1 " 0.9
277.9 " 2.0
264.9 " 2.0
259.9 " 2.4
264.6 " 2.6
262.4 " 3.4
254.3 " 3.1
265.5 " 4.6
255.0 " 4.8
275.9 " 5.1
252.8 " 2.7
259.5 " 5.0
258.5 " 5.3
263.4 " 7.1
5
5
a
a
2
3
4
3
4
3
2
3
4
4
3
-H C-c-C H NO
3 5 4
-H C-c-C H NO
3
5
4
-H C-c-C H NO
90.9 " 3.3
3
5
4
-NC-c-C H NO
272.8 " 2.4
267.2 " 4.5
y49.5 " 4.5
y269.7 " 4.4
y225.1 " 2.2
y245.1 " 1.8
126.0 " 0.9
87.2 " 3.9
5
4
Ž22.
-NC-c-C H NO
283.5 " 1.1
5
4
Ž22.
-HO-c-C H NO
y43.7 " 1.7
y243.0 " 2.6
y221.5 " 1.5
y234.8 " 4.7
135.4 " 5.2
5
4
-HO C-c-C H NO
2
5
4
Ž1.
Ž1.
Ž1.
-HO C-c-C H NO
2
5
4
-HO C-c-C H NO
2
5
4
b
b
-O N-c-C H NO
2
5
4
-H C-4-O N-c-C H NO
101.4 " 6.0
3
2
5
3
a
b
TiCl3 reduction method; all others are combustion measurements.
Estimated value.
case as part of a 5-membered ring. Theoretical SCF-CI calculations by Shahawy et
Ž24.
al.
indicate an intramolecular hydrogen bond in this molecule, and, as can be
seen in table 3, the enthalpies of sublimation of both 2CO HPyNO and 2CO HPy
2
2
are relatively small when compared with the values for isomers, which is an
additional indication of intramolecular hydrogen bonding in these molecules.
y1
.
Hence the difference of ca. 20 kJ mol can be regarded as increase in energy of
the intramolecular hydrogen bond in 2CO HPyNO over that in 2CO HPy.
2
2
T
m
The approximate constancy of D ŽN-O. in pyridine N-oxide derivatives arises
because the effect on ⌬ H Žg. of substitution into pyridine is matched by the
T
m
f
corresponding substitution into pyridine N-oxide. Tables 5 and 6 show the effect of
substitution into pyridine N-oxide and pyridine, respectively, compared with
T
TABLE 5. Standard molar enthalpy changes ⌬ H at T s 298.15 K Ž pЊ s 0.1 MPa.
r
m
for comparison of substitution into pyridine N-oxide with benzene
R-c-C H NOŽg. q C H Žg. s c-C H NOŽg. q R-C H Žg.
5
4
6
6
5
5
6
5
y⌬ H rŽkJ .moly1.a
T
r
m
Position of substituent
R
2
3
4
-
-
-
-
-
CH 3
2.7 " 2.7
16.4 " 5.0
0.2 " 2.9
y28.2 " 3.4
y16.2 " 3.4
y6.0 " 4.8
0.3 " 3.6
CO H
y8.2 " 3.1
2
CN
OH
NO2
y10.7 " 5.1
y17.7 " 1.7
a
T
m
The ⌬ H values were calculated using auxiliary data for C H Žg. and R-C H Žg.
r
6
6
6
5
Ž22.
listed by Pedley et al.
table 4.
Values for R-c-C H NOŽg. and c-C H NOŽg. are listed in
5 4 5 5

T
m
D
ŽN-O. in pyridine N-oxides
877
T
TABLE 6. Standard molar enthalpy changes ⌬ H at T s 298.15 K Ž pЊ s 0.1 MPa.
r
m
for comparison of substitution into pyridine with benzene
R-c-C H NŽg. q C H Žg. s c-C H NŽg. q R-C H Žg.
5
4
6
6
5
5
6
5
y⌬ H rŽkJ .moly1.a
T
r
m
Position of substituent
R
2
3
4
-
-
-
-
-
-
CH 3
9.1 " 1.3
6.7 " 3.5
y7.1 " 2.7
41.1 " 1.9
26.8 " 1.7
1.9 " 1.4
y14.8 " 2.8
y4.3 " 3.0
5.1 " 2.0
4.2 " 1.4
y1.5 " 5.3
y9.9 " 2.5
2.2 " 2.4
15.0 " 1.9
y10.1 " 5.3
CO H
2
CN
OH
NH 2
NO2
0.7 " 2.1
b
a
T
m
The ⌬ H values were calculated using auxiliary data for C H Žg. and R-C H Žg.
r
6
6
6
5
Ž22.
listed by Pedley et al.
. b
Values for R-c-C H NŽg. and c-C H NŽg. are listed in table
5 4 5 5
4
Estimated value.
substitution into benzene. It is apparent that the general pattern of effects is
similar in each case so that values in table 6 could assist in prediction of the
enthalpies of formation of the corresponding pyridine N-oxide derivatives.
Thanks are due to Junta Nacional de Investiga ¸c ˜a o Cient ´ı fica e Tecnol ´o gica,
JNICT, for financial support through Centro de Investiga ¸c ˜a o em Qu ´ı mica da
Universidade do Porto ŽLinha 5. and for the research project PRAXIS XXI
Žproject 2r2.1rQUIr54r94..
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(Recei¨ed 9 October 1997; in final form 3 February 1998)
WA97r070