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Thermodynamic Investigation of Enol↔Keto Tautomerism for AlcoholSensors Based on Carbon Nanotubes as Chemical SensorsM. Monajjemi a; L. Mahdavian b; F. Mollaamin c; B. Honarparvar a

a Department of Chemistry, Science & Research Branch, Islamic Azad University, Tehran, Iran b

Department of Chemistry, Islamic Azad University, Doroud, Iran c Department of Chemistry, QomBranch, Islamic Azad University, Qom, Iran

Online publication date: 15 January 2010

To cite this Article Monajjemi, M., Mahdavian, L., Mollaamin, F. and Honarparvar, B.(2010) 'ThermodynamicInvestigation of Enol↔Keto Tautomerism for Alcohol Sensors Based on Carbon Nanotubes as Chemical Sensors',Fullerenes, Nanotubes and Carbon Nanostructures, 18: 1, 45 — 55To link to this Article: DOI: 10.1080/15363830903291564URL: http://dx.doi.org/10.1080/15363830903291564

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Thermodynamic Investigation of Enol «KetoTautomerism for Alcohol Sensors Based on Carbon

Nanotubes as Chemical Sensors

M. Monajjemi,1 L. Mahdavian,2 F. Mollaamin,3 and B. Honarparvar1

1Department of Chemistry, Science & Research Branch, Islamic Azad University,

Tehran, Iran2Department of Chemistry, Islamic Azad University, Doroud Branch, Doroud,

Iran3Department of Chemistry, Qom Branch, Islamic Azad University, Qom, Iran

Abstract: Single-walled carbon nanotubes (SWNT) are molecular-scale wires with

high mechanical stiffness and strength. The faster response and substantially

higher sensitivity of chemical nanotubes sensors over existing solid-state sensors,

which was due to exposure of gaseous molecules such as C2H5OH, served as the

basis to consider gas sensors based on SWNT. These factors also focused on the

ethanol sensing effects of the sensors on the acetaldehyde and ethyl methyl ketone

tautomerism at room temperature, which have been formed within alcohol

dehydrogenation and condensation reactions involved in chain growth pathways

on SWNT.

Since temperature has a strong effect on structure and stability, we were

interested in calculating some specific thermo chemical properties to justify the

structural stability of existing isomers in keto«enol tautomerism. So, the

thermodynamic and electric properties of produced ethyl-methyl ketone involved

in keto«enol tautomerism have been investigated using semi-empirical methods.

The obtained results showed that the keto structure is more stable because of

intramolecular hydrogen bond formation due to strong C5O bond existence in

the keto tautomers. Our finding implies that the semi-empirical quantum

chemical calculations can be suggested as proper methods to study the interaction

of different chemical compounds with SWNT in a reliable way.

Keywords: Acetaldehyde, Electrical resistance, Ethanol sensor, Ethyl methyl

ketone, keto and enol tautomerism, Single-walled carbon nanotube, Thermo-

dynamic functions

Address correspondence to Dr. Fatemeh Mollaamin, Department of Chemistry,

Qom Branch, Islamic Azad University, Qom, Iran. E-mail: [email protected]

Fullerenes, Nanotubes and Carbon Nanostructures, 18: 45–55, 2010

Copyright # Taylor & Francis Group, LLC

ISSN 1536-383X print/1536-4046 online

DOI: 10.1080/15363830903291564

45

Downloaded By: [Mollaamin, Fatemeh] At: 17:25 15 January 2010

1. INTRODUCTION

Sensors continue to make a significant impact on everyday life. There has

been a strong demand for producing highly selective, sensitive,

responsive, and cost-effective sensors (1).

Carbon nanotubes (CNTs) have many distinct properties that may

be exploited to develop the next generation of sensors. Utilization of

these properties has led to applications of individual nanotubes or

ensembles of nanotubes as scanning probes (2, 3), electron field emission

sources (4), actuators (5), and nanoelectronic devices (6).

Here we report the realization of individual semiconducting single-

walled carbon nanotube (S-SWNT)-based chemical sensors capable of

detecting small concentrations of alcohol gas molecules through quantum

mechanical calculations of thermochemical and other electronic struc-

tural parameters. The nanotube sensors exhibit a fast response and a

substantially higher sensitivity than that of existing solid-state sensors at

room temperature. Sensor reversibility is achieved by slow recovery

under ambient conditions or by heating to high temperatures (7). The

interactions between molecular species and SWNTs and the mechanisms

of molecular sensing with nanotube molecular wires have been of great

interest to the authors.

Ethanol is the most popular member of the alcohol family because

of its rare blend of useful and harmful effects. This is the only alcohol

most people encounter in their daily lives. Detection and control of

ethanol is necessary for the well being of society. Ethanol sensors are

useful in various areas such as the control of drunken driving and the

monitoring of fermentation and other processes in chemical industries.

Development of ethanol sensors based on thin film technology offers

the advantages of greater sensitivity, shorter response time and lower

costs.

The keto«enol tautomerism is one of the most common investigated

subjects of isomerism that occurs for carbonyl compounds (8–12).

Sensors are devices that detect or measure physical and chemical

quantities such as temperature, pressure, sound and concentration. The

measured are converted into an electrical signal. The main requirements

of a good sensor are high sensitivity, fast response, low cost, high volume

production and high reliability (see Figure 1).

In this work, gas sensors based on SWNT have been considered, and

we focused on the ethanol sensing effects of the sensors on the

acetaldehyde and ethyl methyl ketone tautomerism at room temperature.

Generally, the keto form is more stable than the enol form for neutral

systems. The performed calculations illustrated the effects of the metallic

additives and of the adsorbed molecule on the grain binding strength and

its conductance.

46 M. Monajjemi et al.


2. COMPUTATIONAL METHODS

Semi-empirical calculations are set up with the same general structure as

HF calculations in that the former have a Hamiltonian and a wave

function. Within this framework, certain pieces of information are

approximated or completely omitted. Usually, the core electrons are not

included in the calculation, and only a minimal basis set is used. Also,

some of the two-electron integrals are omitted. To correct for the errors

introduced by omitting part of the calculation, the method is

parameterized (12).

The advantage of semi-empirical calculations is that they are much

faster then ab initio calculations. The modified neglect of diatomic

overlap (MNDO) method has been found to give reasonable qualitative

results for many organic systems (12).

Transport properties can be expressed in terms of the submatrix S12

describing transmission through the wire. For example, the electrical

conductance G is given by the Landauer formula (13) as:

G~2e2

h

ðdE

{Lf Eð ÞLE

Tr S{12 Eð ÞS12 Eð Þ

n oð1Þ

where f(E) is the Fermi–Dirac distribution function and S(E) is the

scattering matrix. Similarly, thermodynamic properties can be expressed

in terms of the scattering matrix through the electronic density of states

(DOS) D(E) (13) as:

D Eð Þ~ 1

2piTr S{ Eð Þ LS

LE{

LS{

LES Eð Þ

� �ð2Þ

from which the relevant thermodynamic potential for an open system,

namely, the grand canonical potential V, is obtained as (13):

V~{kBT

ðdED Eð Þ ln 1ze

{E{mð ÞkBT

h ið3Þ

Figure 1. Cross sectional structure of the SWNT field effect transistors (FET-

based sensor) and the experimental geometry (7).

Alcohol Sensors Based on Carbon Nanotubes 47


where kB is the Boltzmann constant, T is the temperature and m the

chemical potential specified by the macroscopic electrodes.The geometry optimizations were performed with semi-empirical

calculations using the Gaussian A7 package (14). The accuracy of the

semi-empirical quantum mechanics method depends on the database

used to parameterize the method. We can use the information obtained

from semi-empirical calculations to investigate many thermodynamic and

kinetic aspects of chemical processes. Energies and geometries of

molecules have clear relationships to chemical phenomena (15, 16).

3. RESULTS AND DISCUSSION

Semiconducting sensors offer an inexpensive and simple method for

monitoring gases. The change of the electrical conductivity of semi-

conducting materials upon exposure to reducing gas C2H5OH has been

used for gas detection. Therefore, CNTs have been used as sensingmaterials in pressure, flow, thermal, gas, optical, mass, position, stress,

strain, chemical and biological sensors (17–19). The sensor was based on

the principle that the electronic properties of CNTs change when

subjected to strains. The isotropic nature of CNT films helps in

measuring strains in multiple locations and in different directions. This

study demonstrated using first-principle quantum transport calculations,

semi-empirical with MNDO method that hydrostatic pressure can induce

radial deformation and, therefore, electrical transition of SWNTs. Themeasurements were carried out in a humid air atmosphere, which is the

same condition of general operation of a sensor.

3.1. Interaction of Ethanol with SWNT

We have investigated the formation of the two products observed most

commonly in significant amounts, namely, acetaldehyde, Ethyl methylketone as follows:

The reaction between the reducing gases led to the decrease of the

carrier-hole density in the surface charge layer and thus the increase in

the SWNT resistance. Also, the reduction in tube length and diameter

enhances the sensitivities of sensors (20), in this study, we have

considered SWNT armchair (4,4).

3.2. Acetaldehyde Formation

The characteristics of sensor have been investigated by changing of

resistance in ethanol with different products in air. Formation of



acetaldehyde by the oxidative dehydrogenation of ethanol depends upon

the reaction step that requires the oxide surface to acquire a negative

charge; in Figure 2, the ball stick model shows this reaction. It is well

accepted that upon adsorption the O–H bond of the alcohol dissociates

hydrolytically to yield an ethoxide and a hydrogen molecule as follows

(20):

When ethanol is injected into the micro-reactor containing either pure

SWNT compound a large fraction of ethanol that was found to be

transformation to acetaldehyde. It then may be adsorbed in the form of

ethoxide (CH3CH2O), producing an adsorbed hydrogen atom. This

transformed to acetaldehyde (CH3CHO) forming an adsorbed hydrogen

atom (20).

CH3CH2OH gð Þ?CH3CH2OadszHads

CH3CH2Oads?CH3CHOadszHads

HadszHads?2Hzadsz2e

The adsorbed hydrogen atoms then loose electrons to the conduction

band of SWNT that electrical transition of SWNTs were increased. The

generated protons (H+) join together and eliminate H2. During the above

dehydrogenation process, one net electron is released into the conduction

band of SWNT, reducing its resistance. The graph of electrical resistance

(V) of ethanol and acetaldehyde formation using SWNT is displayed in

Figure 2. Schematic diagram of the CNT array ethanol, acetaldehyde formation.



Figure 3. As can be seen, the electrical resistance (V) of ethanol and

acetaldehyde formation on SWNT has been increased from 20.1 to 2.0.2

in time intervals from 0 to 2.5. This increasing trend is due to the

generated electrons from the adsorbed hydrogen atoms to the conduction

band of SWNT that led to increase in electrical transition of SWNTs.

After that it remains constant for some time and then decreased in

reference to product formation of considered reaction.

The thermodynamic properties related to this interaction between the

molecules of ethanol and carbon nanotubes have been calculated through

quantum mechanical calculations of IR vibrational frequencies imple-

mented in G98 program using semi-impractical method as listed in

Table 1. The heat formation energy for this interaction is exothermic,

increasing to 6.20 MJ/mol and then decreasing to 5.90 MJ/mol and

yielding the lowest value.

3.3. Ethyl Methyl Ketone Formation

In the environment of reaction, there are ethanol and acetaldehyde where

ethyl methyl ketone represents the second most important reaction

product in our calculations. This reaction requires proton transfer from

one adsorbed acetaldehyde, which becomes oxidized, to another

adsorbed ethanol, which is reduced to alkoxide. The thermodynamic

parameters of interaction between ethanol (ethanol and acetaldehyde on

SWNT (4,4)-based sensor to ethyl methyl ketone) and acetaldehyde using

SWNT (4,4) have been reported in Table 2. According to the results,

Figure 3. The graph of electrical resistance (V) of ethanol and acetaldehyde

formation versus time (s) using SWNT.



along with transition from step 1 up to step 15, the more negative values

have been yielded for E total and Gelec which shows the exotermic

characteristic of considered reaction.

This process may form a complex in a transition state that requires

participation of the surface oxygen: where an adsorbed acetaldehyde

molecule with the participation of a surface oxygen anion transfers a

hydride to another adsorbed ethanol molecule, which has been shown in

Figure 4. In the sensor, we have set the sensor signals based on the

relative changes of the electrical resistance (V) so we had to convert

calculated data (see Table 2) to the electrical resistance (V) shown in

Figure 5.

Table 1. The thermodynamic properties of interaction between ethanol (ethanol

on SWNT (4,4)-based sensor to acetaldehyde) with SWNT (4,4)

Steps E total

(MJ/mol)

Eelec

(V)

E bin

(MJ/mol)

H

(MJ/mol)

Gelec

(MJ/mol)

E nuc

(MJ/mol)

1 21046.14 29168.12 250.87 7.05 215604.09 14557.95

2 21109.01 29769.32 252.16 6.26 216627.33 15518.32

3 21173.11 210576.2 255.20 6.21 218000.66 16827.56

4 21173.23 210578.8 255.33 6.09 218005.10 16831.87

5 21173.12 210690.5 255.22 6.20 218195.23 17022.10

6 21173.40 210684.9 255.50 5.92 218185.71 17012.31

7 21173.42 210654.3 255.52 5.90 218133.50 16960.08

Table 2. The thermodynamic properties of interaction between ethanol

(Ethanol and acetaldehyde on SWNT (4,4)-based sensor to ethyl methyl

ketone) and acetaldehyde using SWNT (4,4)

Steps E total

(MJ/mol)

Eelec

(V)

E bin

(MJ/mol)

H

(MJ/mol)

Gelec

(MJ/mol)

E nuc

(MJ/mol)

1 21046.14 29168.12 250.87 7.05 215604.09 14557.95

2 21109.01 29769.32 252.16 6.26 216627.33 15518.32

3 21147.88 210456.58 232.81 30.70 217797.05 16649.17

4 21173.57 210777.76 258.50 4.98 218343.70 17170.14

5 21172.66 210836.47 257.59 5.89 218443.62 17270.96

6 21173.39 210910.52 258.32 5.15 218569.65 17396.26

7 21171.85 210949.36 256.78 6.70 218635.75 17463.9

8 21173.40 210907.19 258.33 5.15 218563.98 17390.58

9 21172.13 210972.07 257.06 6.41 218674.41 17502.28

10 21174.27 210952.52 259.20 4.28 218641.13 17466.87

11 21173.70 210998.95 258.63 4.85 218720.17 17546.47

12 21172.15 210934.97 257.09 6.39 218611.28 17439.12

13 21172.01 210847.56 256.94 6.54 218462.49 17290.49

14 21172.31 210607.50 257.25 6.23 218053.92 16881.61

15 21172.65 210613.79 257.59 5.89 218064.62 16891.96



A current versus voltage curve recorded with a SWNT sample after

time exposure to ethanol showed upfold conductance depletion.

Exposure to ethanol molecules increased the conductance of the SWNT

sample. The SWNT is a hole-doped semiconductor, as can be gleaned

from the current versus gate voltage curve shown in Figure 5 (middle

curve), where the resistance of the SWNT is observed to be decreased.

Figure 4. Schematic diagram of the CNT array ethanol, ethyl methyl ketone

formation.

Figure 5. Electrical resistance (V) of from ethanol with SWNT and ethyl methyl

ketone formation.



Tautomerism refers to the equilibrium between two different

structures of the same compound. Usually the tautomer forms differ in

the point of attachment of a hydrogen atom. One of the most common

examples of a tautomeric system is the equilibrium between a ketone and

its enol form due to flexibility of the C5C—O chain in the enol form and

convergence of the C—C5O chain in the keto:

This result revealed a weakly formed enol species in proximity of an

ethoxide species in an unstable configuration. Both species react together

and yielded an ethyl methyl ketone molecule. Therefore, it can be

deduced that ethyl methyl ketone has a more active surface reaction than

ethanol on SWNT and results in higher sensitivity and a quicker response

time.

3.4. The Other Thermodynamic Properties

Thermodynamic equilibrium constants (K) for these interactions were

calculated by the related standard Gibbs free energy difference (DGelec):

K~exp {DGelec=RTtrð Þ ð1Þ

where Ttr is the transition temperature and R is gas constant. According

to our results, the keto structure is the most stable form in ethyl methyl

ketone, but the enol structure is the most stable form for it keto « enol

tautomerism (20). The equilibrium constant can be defined as follows:

K~keto½ �enol½ � ð2Þ

where [enol] and [keto] represent the molar concentration of the enol and

keto tautomers, respectively. The entropy difference (DS) at the phase

transitions may be given by:

DS~DG

Ttr

ð3Þ

where DH is the electronic enthalpy difference (20, 21). The thermo-

dynamic properties of interaction of ethanol on SWNT (4,4)-based



sensor have been listed in Table 3. Based on these obtained thermo-

dynamic parameters, the conformational preference or the relative

stability of tautomers can be distinguished. According to the reported

data, it can be inferred that the equilibrium constant (K) of the second

reaction (CH3CH2COCH3 + H2O), which contains the keto form is much

higher than other cases (K57.99 61046). In addition, the most negative

free Gibs energy (DG5 2267.57 MJ/mol) and the most negative enthalpy

(DH5 224.77MJ/mol) as well as the most positive entropy (DS5

0.81MJ/mol) was related to this case.

4. CONCLUSION

The current research sets a goal to investigate the interactions between

the molecules of ethanol and carbon nanotubes. In this work, gas sensors

based on SWNT-based chemical sensors capable of detecting small

concentrations of alcohol gas molecules have been considered. We

focused on the ethanol sensing effects of the sensors on the acetaldehyde

and ethyl methyl ketone tautomerism through using semi-empirical

calculations of thermochemical and other electronic structural para-

meters of produced ethyl-methyl ketone involved in keto«enol

tautomerism. These calculations have been carried out to find the most

preferable tautomer in keto«enol tautomerism which fulfills the

structural stability.

The sensitivity of carbon nanotubes is affected by ethanol

concentration at specific room temperature. The sensitivity increases

with increasing ethanol gas concentration. The response and sensitivity of

the alcohol-treated sensor were found to be very high compared with the

air-treated sensor.

Development of ethanol sensors based on thin film technology offers

the advantages of greater sensitivity, shorter response time and lower

costs. The sensors developed with alcohols were found to have good

selectivity and sensitivity and were unaffected by various environmental

conditions.

Table 3. The thermodynamic properties of interaction of ethanol on SWNT (4,

4)-based sensor

Products DG0elec

(MJ/mol)

K DH

(MJ/mol)

DS

(MJ/mol)

CH3CHO+H2 904.13 3.36102159 5.61 23.02

CH3CH2COCH3+H2O 2267.57 7.9961046 224.77 0.81

enol«keto tautomerism 263.13 1.1761011 20.02 0.21



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