Energetics of 6-methoxyquinoline and 6-methoxyquinoline N-oxide: The dissociation enthalpy of the (N-O) bond

Article abstract of DOI:10.1016/S0021-9614(03)00052-1

The standard (p° = 0.1MPa) molar enthalpies of formation, at T = 298.15K, were determined using static-bomb calorimetry for crystalline 6-methoxyquinoline N-oxide (6MeOQUINO), as -(85.6 ± 3.7) kJ·mol^-1 and liquid 6-methoxyquinoline (6MeOQUI), -(25.8 ± 2.2)kJ·mol^-1. The standard molar enthalpies of sublimation/vaporization, at T = 298.15K, were measured by microcalorimetry and from the enthalpies of formation of the gaseous compounds, the dissociation enthalpy D_m⁰ of the (N⁺-O^-) dative covalent bond was derived, as (269.2 ± 5.0)kJ·mol^-1.

Full text of DOI:10.1016/S0021-9614(03)00052-1

J. Chem. Thermodynamics 35 (2003) 1093–1100
www.elsevier.com/locate/jct
Energetics of 6-methoxyquinoline and
-methoxyquinoline N-oxide: the dissociation
enthalpy of the (N–O) bond
6
a,
a
*
Maria D.M.C. Ribeiro da Silva , Lu ꢀı s M.N.B.F. Santos ,
a
Ana L.R. Silva , Oscar Fernandes , W.E. Acree Jr.
a
b
ꢀ
a
Centro de Investigacß a~ o em Qu ꢀı mica, Departamento de Qu ꢀı mica, Faculdade de Ci e^ ncias,
Universidade do Porto, Rua do Campo Alegre, 687, P-4169-007 Porto, Portugal
b
Department of Chemistry, University of North Texas, Denton, TX 76203-5070, USA
Received 16 September 2002; accepted 25 February 2003
Abstract
o
The standard ðp ¼ 0:1MPaÞ molar enthalpies of formation, at T ¼ 298:15K, were deter-
mined using static-bomb calorimetry for crystalline 6-methoxyquinoline N-oxide (6MeOQU-
À1
INO), as Àð85:6 Æ 3:7Þ kJ Á mol and liquid 6-methoxyquinoline (6MeOQUI), Àð25:8 Æ 2:2Þ
À1
kJ Á mol . The standard molar enthalpies of sublimation/vaporization, at T ¼ 298:15K, were
measured by microcalorimetry and from the enthalpies of formation of the gaseous com-
À
0
m
þ
pounds, the dissociation enthalpy D of the ðN –O Þ dative covalent bond was derived, as
À1
ð269:2 Æ 5:0ÞkJ Á mol
.
Ó 2003 Elsevier Science Ltd. All rights reserved.
1
. Introduction
The discovery of attractive functional group interactions in solids aids the devel-
opment of crystal engineering and organic material science. A number of N-oxides
have shown to undergo selective reduction under anaerobic conditions, and are be-
lieved to be likely bioreducing agents able to act selectively on hypoxic cells of solid
*
Corresponding author. Fax: +351-2260-82-822.
E-mail address: mdsilva@fc.up.pt (M.D.M.C. Ribeiro da Silva).
0
021-9614/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0021-9614(03)00052-1

1094
M.D.M.C. Ribeiro da Silva et al. / J. Chem. Thermodynamics 35 (2003) 1093–1100
tumours. In this context, formation of N-oxide derivatives has been attracting the
interest from many viewpoints that include very important pharmacological applica-
tions for which the basic knowledge of chemical bonding has to be expanded [1–4].
The thermochemical study of the energetics of the (N–O) bond in some classes of
N-oxide derivatives has been justified by the variation of the dissociation enthalpy of
the (N–O) dative covalent bond with the molecular environment of the bond. The
0
m
effect of substitution of various groups into the pyridinic ring upon D ðN–OÞ has
been already investigated for several pyridine N-oxides [5–7] but there are not any
results for quinoline N-oxide derivatives.
This paper reports experimental results that allow to derive the values of the stan-
dard molar enthalpies of formation, in the gaseous state, for 6-methoxyquinoline N-
oxide and 6-methoxyquinoline. With the knowledge of those data, the first value for
the enthalpy of dissociation of the (N–O) bond in a quinoline N-oxide is derived.
2
. Experimental
-Methoxyquinoline (6MeOQUI) obtained from Aldrich was purified by repeat-
edly fractional distillation under reduced pressure. Purity was checked by g.l.c. (mass
6
À3
fraction of impurities was <2 Á 10 , the retention time, 5.9 min, was measured by gas
chromatography, using an HP 4890 apparatus with a column HP-5, 5% diphenyl and
9
5% dimethylpolysiloxane, under a nitrogen pressure of 21 kPa and oven tempera-
ture 423 K) as well as the carbon dioxide recovery ratio for combustion experiments.
Since the compound is hygroscopic, freshly distilled material was used in all the ex-
perimental measurements.
6-Methoxyquinoline N-oxide (6MeOQUINO) was prepared by oxidation of
MeOQUI with an acetic acid–hydrogen peroxide mixture according to published
6
methods for synthesizing substituted pyridine N-oxides, quinoline N-oxides, pyr-
azine N-oxides and quinoxaline N-oxides [8–11]. The crude product was crystallized
three times from hot benzene. The melting temperature of the purified compound
ð377:0 Æ 0:5KÞ was in excellent agreement with literature value 376 K to 378 K
[
12]. Elemental analysis were in excellent agreement with expected values, and the
compositions in mass fractions w of C, H and N were as follows for C10
9 2
H NO ,
found: wðCÞ, 0.6862; wðHÞ, 0.0515 and wðNÞ, 0.0801; calculated: wðCÞ, 0.6856;
wðHÞ, 0.0518 and wðNÞ, 0.0800.
The enthalpy of combustion of 6MeOQUI was determined with an isoperibol sta-
3
tic bomb calorimeter, with a twin valve bomb of internal volume of 0:290dm , that
has been used formerly at the National Physical Laboratory, Teddington [13], and in
Manchester [14]. This calorimeter has been transferred from Manchester to Porto
and was used mainly as previously described [13,14], but with a few changes in tech-
nique because of different auxiliary equipment used. The electrical energy for ignition
was determined from the change in potential across a 1400lF condenser on
discharge through the platinum ignition wire. The calorimeter temperature was
À4
measured to Æ10 K with a S10 four wire calibrated ultra-stable thermistor
probe (Thermometrics, standard serial No. 1030) and recorded by a high sensitivity

M.D.M.C. Ribeiro da Silva et al. / J. Chem. Thermodynamics 35 (2003) 1093–1100
1095
nanovolt/microohm meter (Agilent 34420 A) interfaced to a microcomputer pro-
grammed to compute the adiabatic temperature change. Temperature measurements
were automatically collected every 10 s and the initial temperature of the combustion
experiments was 298.15 K. The energy of reaction was always referred to the initial
temperature of 298.15 K. The calorimetric system was calibrated using benzoic acid
(
a
Bureau of Analysed Samples, Thermochemical Standard, BCS-CRM 190-p) having
massic energy of combustion under standard bomb conditions of
À1
Àð26; 431:8 Æ 3:7ÞJ Á g . The calibration results were corrected to give the energy
equivalent eðcalÞ corresponding to an average mass of water added to the calorimeter
of 2900.0 g. One set of 12 calibration experiments was made in oxygen at
3
p ¼ 3:04MPa, with 1:00cm of water addded to the bomb, leading to an energy
À1
equivalent of the calorimeter eðcalorÞ ¼ ð15;543:3 Æ 0:6ÞJ Á K , where the uncer-
tainty quoted is the standard deviation of the mean. To check the accuracy of the
apparatus, a series of six experiments was also performed, by burning urea (NBS,
Standard Reference Material 2152) in the presence of an auxiliary of combustion
(
benzoic acid, BCS-CRM 190-r, having a massic energy of combustion under stan-
À1
dard bomb conditions of Àð26; 432:3 Æ 3:8ÞJ Á g ).
The samples of 6MeOQUI, enclosed in Melinex bags, were ignited at
3
T ¼ ð298:150 Æ 0:001Þ K, in oxygen at p ¼ 3:04MPa, with 1cm of water added to
0
À1
the bomb. For dry Melinex, D
c
u ¼ Àð22;902 Æ 5ÞJ Á g [15], this value has been
previously confirmed by combustion 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 [15].
For the cotton thread fuse of empirical formula CH1:686
0
O
0:843, ÀD
c
u ¼
À1
1
6;250J Á g [16], a value previously confirmed in our laboratory. Corrections
À1
for carbon formation were based on À33kJ Á g for the massic energy of combus-
tion of carbon [16]. Corrections for nitric acid formation were based on
À1
À3
À59:7kJ Á mol for the molar energy of formation of 0:1mol Á dm HNO ðaqÞ
3
from N , O , and H OðlÞ [17]. At T ¼ 298:15K, ðou=opÞ for this liquid was assumed
2
2
2
T
À1
À1
to be À0:3J Á g Á MPa , a typical value for organic liquids.
The enthalpy of combustion of 6MeOQUINO was measured by using a static
bomb calorimeter [18,19], with a Parr 1105 model bomb. The energy equivalent of
the calorimeter was determined from the combustion of benzoic acid (Bureau of Anal-
ysed Samples, BCS-CRM 190-p) having a massic energy of combustion under stan-
À1
dard bomb conditions of Àð26;431:8 Æ 3:7ÞJ Á g . From 9 calibration experiments
À1
eðcalorÞ ¼ ð15;908:3 Æ 1:0ÞJ Á K for an average mass of water added to the calorim-
eter of 3119.6 g, where the uncertainty quoted is the standard deviation of the mean.
Samples of 6MeOQUINO, in pellet form, were ignited at T ¼ ð298:150 Æ 0:001ÞK
3
in oxygen at a pressure p ¼ 3:04MPa with a volume of 1:00cm of water added to
the bomb. The electrical energy for ignition was determined from the change in po-
tential difference across a capacitor when discharged through the platinum ignition
wire. The corrections for the cotton thread fuse, carbon formation and nitric acid
formation were made as previously described. At T ¼ 298:15K, ðou=opÞ for this so-
T
À1
À1
lid was assumed to be À0:2J Á g Á MPa , a typical value for organic solids.

1096
M.D.M.C. Ribeiro da Silva et al. / J. Chem. Thermodynamics 35 (2003) 1093–1100
0
For both compounds, the values of ÀD
c
u were calculated by the procedure given
by Hubbard et al. [20]. The relative atomic masses used throughout this paper were
those recommended by the IUPAC Commission in 1999 [21]. The amount of sub-
stance used in each experiment was determined from the total mass of carbon diox-
ide produced after allowance for that formed from the cotton thread fuse and
Melinex and that lost due to carbon formation. The average ratios, hri of the mass
of carbon dioxide produced by the samples during the energy of combustion mea-
surement to that calculated from its mass with uncertainties of twice the standard
deviation of the mean were: for 6MeOQUINO, hri ¼ ð0:9950 Æ 0:0006Þ, assuming
q ¼ 1:154; for 6MeOQUI, hri ¼ ð0:9990 Æ 0:0005Þ, q ¼ 1:154 [22]. These results re-
flect that both compounds are hygroscopic.
The enthalpy of sublimation of 6MeOQUINO was measured by the ‘‘vacuum
sublimation’’ drop microcalorimetric method [23]. Samples, about 5 mg of com-
pound, contained in a thin glass capillary tube sealed at one end were dropped at
room temperature into the hot reaction vessel in the Calvet High Temperature Mi-
crocalorimeter (SETARAM HT 1000), held at the T ¼ 383K, and previously filled
with dry nitrogen, and then removed from the hot zone by vacuum sublimation.
0
The observed standard molar enthalpies of sublimation fH ðg; 383KÞÀ
m
0
m
383 K
298:15 K
0
H ðcr; 298:15KÞg were corrected to T ¼ 298:15K using D
H ðgÞ ¼ 17:0kJÁ
m
À1
mol estimated by a group method based on the values of Stull et al. [24]. The mi-
crocalorimeter was calibrated in situ for these measurements by making use of the
reported molar enthalpy of sublimation, at T ¼ 298:15K, of naphthalene, C H :
10
8
À1
ð72:51 Æ 0:01ÞkJ Á mol [25]. The enthalpy of vaporization of 6MeOQUI was also
measured, at T ¼ 383K, using a similar method [26] to that used for 6MeOQUINO
3
83 K
0
m
À1
and the estimated correction D2
the microcalorimeter was also calibrated in situ, using the reported molar enthalpy of
H ðgÞ ¼ 15:8kJ Á mol ; for these experiments,
98:15 K
vaporization, at T ¼ 298:15K, of undecane: ð56:77 Æ 0:83ÞkJ Á molÀ1 [27].
3
. Results
Results for a typical combustion experiment on each compound, including urea
used as a test material, are given in table 1; DmðH
of water added to the calorimeter from 3119.6 g or 2900.0 g, the mass assigned for
eðcalorÞ depending of the apparatus used; DU is the correction to the standard state;
the remaining terms are as previously described [20]. As samples were ignited at
2
OÞ is the deviation of the mass
R
T ¼ 298:15K, DUðIBPÞ ¼ ÀfeðcalorÞ þ c
p
ðH
2
O; lÞ Á DmðH
2
OÞ þ e gDTad þ DUðignÞ,
f
where DT is the calorimeter temperature change corrected for heat exchange and
ad
0
the work of stirring. The individual values of ÀD u , together with the mean and
c
its standard deviation, are given in table 2. Table 3 lists the derived standard molar
enthalpies of combustion and of formation, in the condensed and gaseous states,
for 6MeOQUI and 6MeOQUINO. In accordance with normal thermochemical prac-
tice, the uncertainties assigned to the standard molar enthalpies of combustion and
formation are twice the overall standard deviation of the mean and include the uncer-
0
0
tainties in calibration and in the auxiliary quantities used. To derive D
f
H from D H
m
c
m

M.D.M.C. Ribeiro da Silva et al. / J. Chem. Thermodynamics 35 (2003) 1093–1100
1097
TABLE 1
Typical combustion experiments for 6-methoxyquinoline N-oxide, 6-methoxyquinoline and urea, at
T ¼ 298:15K
6
MeOQUINO
6MeOQUI
Urea
mðCO
2
Þ=g
1.77521
0.70520
–
2.16138
–
0
a
m ðcompoundÞ=g
0.74587
0.04081
–
0.73452
–
0
0
m ðMelinexÞ=g
0
00
m ðbenzoic acidÞ=g
–
0.41752
0.00380
1.21624
0
00
m ðfuseÞ=g
0.00224
1.30308
15.82
0.1
0.00361
1.63218
15.96
DTad=K
À1
e
f
=ðJ Á K
Þ
16.91
1.0
DmðH
2
OÞ=g
)1.9
25,381.43
b
ÀDUðIBPÞ=J
20,749.84
34.01
1.11
18,929.19
70.69
0.84
DUðHNO
DUðignÞ=J
DU =J
3
Þ=J
38.63
1.11
16.75
R
14.28
–
19.82
–
DUðMelinexÞ=J
DUðbenzoic acidÞ=J
DUðfuseÞ=J
934.58
–
58.63
–
11,036.05
61.71
36.38
29,303.98
0
À1
ÀD
c
u =J Á g
32,623.43
10,538.75
a
2
The mass of the compound has been determined by the CO recovery analysis.
b
DUðIBPÞ already includes the DUðignÞ.
TABLE 2
0
Individual values of the massic energy of combustion, D
c
u , of 6-methoxyquinoline N-oxide, 6-methoxy-
quinoline and urea, at T ¼ 298:15K
u =ðJ Á gÀ1Þ
0
ÀD
c
6MeOQUINO
6MeOQUI
Urea
2
2
2
2
2
2
9,303.98
9,297.79
9,263.18
9,317.34
9,320.05
9,326.67
32,623.43
32,627.77
32,632.45
32,601.12
32,616.62
32,606.93
10,548.21
10,527.48
10,549.49
10,538.75
10,532.07
10,524.52
ÀhD
c
u i=ðJ Á gÀ1Þa
0
2
9; 304:8 Æ 9:4
32; 618:0 Æ 5:0
10; 536:8 Æ 4:3
a
Mean value and standard deviation of the mean.
TABLE 3
o
0
m
Derived standard ðp ¼ 0:1MPaÞ molar energies of combustion, D
c
U , standard molar enthalpies of com-
0
m
g
0
bustion, D
c
H , standard molar enthalpies of sublimation/vaporization, D H , and standard molar
cr;l
m
0
m
enthalpies of formation, D
f
H , for 6-methoxyquinoline N-oxide, 6-methoxyquinoline at T ¼ 298:15 K
0
m
0
m
0
m
g
cr;l
0
m
0
m
D
c
U ðcr; lÞ
ðkJ Á mol
D
c
H ðcr; lÞ
D
f
H ðcr; lÞ
D
H
D
f
H ðgÞ
À1
À1
À1
ðkJ Á molÀ1
Þ
ðkJ Á molÀ1Þ
Þ
ðkJ Á mol
Þ
ðkJ Á mol
Þ
6
MeOQUINO
MeOQUI
À5133:8 Æ 3:4 À5135:7 Æ 3:4
À85:6 Æ 3:7
117:9 Æ 1:0
32:3 Æ 3:8
6
À5192:4 Æ 1:8 À5195:5 Æ 1:8
À25:8 Æ 2:2
78:1 Æ 2:3
52:3 Æ 3:2

1098
M.D.M.C. Ribeiro da Silva et al. / J. Chem. Thermodynamics 35 (2003) 1093–1100
the standard molar enthalpies of formation. For H
À1
2
OðlÞ: Àð285:83 Æ 0:04ÞkJ Á molÀ1
and for CO
2
ðgÞ: Àð393:51 Æ 0:13ÞkJ Á mol , were used [28].
4
. Discussion
The result obtained for the standard massic energy of combustion of urea,
0
À1
D
c
u ¼ Àð10;536:8 Æ 4:3ÞJ Á g , is in very good agreement with the certificated value
0
À1
c
supplied by NIST, D
u ¼ Àð10:536 Æ 0:003Þ MJ Á kg , as well as those reported
0 À1 0
c c
in literature, by Cox ðD u ¼ À10;540J Á g Þ [29] and Johnson ðD u ¼
À1
Àð10;540:6 Æ 2:8ÞJ Á g Þ [30], and that presented as recommended value for the
0
À1
standard massic energy of combustion of urea, D u ¼ Àð10; 539 Æ 5ÞJ Á g , in a re-
c
cent review [31]. The reproducibility and accuracy of the results obtained with the
NPL static bomb calorimeter, transferred from Manchester to Porto, confirm the
good performance of the calorimetric system.
The thermochemical study of the two quinoline and quinoline N-oxide deriva-
tives, particularly the standard enthalpy of formation in the gaseous state, gives
the basis for the knowledge of their thermodynamic behaviour and also of the ener-
getics of the (N–O) bond in 6MeOQUINO. The enthalpy of dissociation of the (N–
0
O) bond, D ðN–OÞ, for 6MeOQUINO corresponds to the enthalpy of the reaction
m
0
m
À1
(
1), requiring also D
f
H ðO; gÞ ¼ ð249:18 Æ 0:10ÞkJ Á mol [28].
6
MeOQUINOðgÞ ¼ 6MeOQUIðgÞ þ OðgÞ
ð1Þ
0
m
À1
The derived value for D ðN–OÞ is ð269:2 Æ 5:0ÞkJ Á mol , which is the first value
for the enthalpy of dissociation of the (N–O) bond in a quinoline N-oxide derivative,
since there are not other values reported before in literature for that class of com-
pounds.
N-oxide chemistry in these bicyclic systems largely parallels the processes de-
scribed for pyridine N-oxide, with the additional possibility of benzene ring electro-
philic substitution. Indeed the value obtained for the enthalpy of dissociation of the
(N–O) bond for 6-methoxy-quinoline N-oxide is in good agreement, within the limits
of uncertainty, with the earlier conclusion [5,6] that the molar dissociation enthalpy
of the (N–O) bond in pyridine N-oxide derivatives lied within the region
À1
ð260 Æ 10ÞkJ Á mol , with exception for the surprising larger values recently ob-
tained for m- and p-pyridine carboxamide N-oxide isomers [7].
It will be interesting to check if in quinoline N-oxide derivatives there will be the
0
approximate constancy of D ðN–OÞ previously found for pyridine N-oxide deriva-
m
0
m
tives [5,6], arising a consequence of the effect on D
f
H ðgÞ of substitution into quin-
oline to be matched by the corresponding substitution into quinoline N-oxide. This
suggests the future study of the tendency of the enthalpy of dissociation of the (N–O)
bond on other quinoline N-oxide derivatives, with substituents in different positions
0
m
in both rings, in order to evaluate their effect on the D ðN–OÞ values for that kind of
molecules. This will imply the thermochemical study of quinoline N-oxide deriva-
tives, since a reasonable number of thermochemical data for quinoline derivatives
is already reported in the literature [32,33].

M.D.M.C. Ribeiro da Silva et al. / J. Chem. Thermodynamics 35 (2003) 1093–1100
1099
Acknowledgements
We thank the Fundaßc ~a o para a Ci ^e ncia e Tecnologia, FCT, Lisbon, for financial
support through Centro de Investigaßc ~a o em Qu ꢀı mica da Universidade do Porto
(
Linha 5).
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