# Computing the Energy and Estrada Index of Different Molecular Structures

Authors: Zeeshan Saleem Mufti; Rukhshanda Anjum; Qin Xin; Fairouz Tchier; Iram Anwar-ul-Haq; Yaé Ulrich Gaba
Journal: Journal of Chemistry
 (2022)

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## Abstract

Graph energy is an invariant that is derived from the spectrum of the adjacency matrix of a graph. Graph energy is actually the absolute sum of all the eigenvalues of the adjacency matrix of a graph i.e.E=∑i=1nλi, and the Estrada index of a graph G is elaborated as EEG=∑i=1neλi, where, λ1,λ2,…,λn are the eigenvalues of the adjacency matrix of a graph. In this paper, energy EG and Estrada index EEG of different molecular structures are obtained and also established inequalities among the exact and estimated values of energies and Estrada index of TUC4C8 nanosheet and naphthalene.


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## Body

## 1. Introduction

Chemical graph theory is a branch of mathematical chemistry and mathematical chemistry gives a natural way to understand the mathematical structures which actually act behind the existing chemical concepts to inaugurate the novel mathematical ideas and mathematical techniques in chemistry. The chemical graph theory is the intersection of graph theory and chemistry. In chemical graph theory, one can pick the molecular structure of different compounds and transform them into a molecular graph and then apply different graph invariants such as the energy of a graph. This graph invariant is much more popular in both mathematicians and chemists and was invented in 1978 by Ivan Gutman [1].When graph energy was introduced in 1978, no one was attracted to this invariant. Even its chemical roots belong to 1940. Mathematicians were not interested initially but when the 20th century was started, a dramatic change occurred and graph energy got great respect from the whole world when the number of countries such as Pakistan, Australia, Austria, Brazil, Canada, and many more produced numbers of papers in graph energy. The definition of graph energy is motivated by already existing results for the Huckel molecular orbital totalπ-electron energy [2, 3]. The energy of a graph defined here totally coincides with the total π-electron energy computed using Huckel molecular orbital theory. The energy of a graph will be zero if the graph is totally disconnected.About 20 years ago Estrada introduced a new index called the Estrada index. The Estrada index is denoted by EE(G) and is defined asEEG=∑i=1neλi [4]. The application of the Estrada index is to measure the folding degree of long-chain proteins [5–8]. To measure the degree of long-chained molecules such as proteins, weighted graphs were used. This index is also used in complex networks to measure the centrality measures of these structures such as social, communication, and metabolic[9–11]. For detailed surveys on the Estrada index, readers can check [4, 12–22].The motivation of the paper was to find the energy and Estrada index of nanostructures. Matrices are used nowadays for dealing with different types of problems in daily life. We are dealing with square matrices which are very useful in different applications such as in the solving system of equations, by using its determinant to find the area and orthogonal vectors. In this paper, we find the energy of a graph by finding the eigenvalues of its adjacency matrix. LetG be a simple undirected graph in which V denotes the vertex set containing v1,v2,…,vn as atoms and E denotes the edge set e1,e2,…,en as bonds. The adjacency matrix AG of a graph G is a square matrix that can be obtained by the adjacent and nonadjacent vertices in such a way if (i, j)- entry of a matrix is 1 whenever the vertices vi and vj are adjacent, otherwise 0. The eigenvalues of the adjacency matrix of order n are λ1,λ2,…,λn and the set of these eigenvalues is called a spectrum of the graph and since the adjacency matrix is symmetric so all the eigenvalues are real.


## 2. Methodology

We follow the following steps to compute the energy and Estrada index of theTUC4C8 nanosheet and naphthalene:(1)
We draw a chemical structure in HyperChem [23]. HyperChem is a molecular editor, which offers a wide range of molecular and quantum mechanics calculations.(2)
The Adjacency matrix is found using TOPOCLUJ [24].(3)
By using MATLAB, the eigenvalues of the adjacency matrix are found [25].(4)
A polynomial of your desired order is constructed which best fits the data using the cf toolbox [26].


## 3. Energy and Estrada Index ofTUC4C8 Nanosheet

Carbon nanosheets(CNTS) belong to a special class of nanomaterials that consist of a two-dimensional hexagonal lattice of carbon atoms. It plays an important role in basic sciences, engineering, and nanotechnology. Carbon nanosheets are not just used in chemistry it is also used in different field of science it is also used in conducting molecules like in semiconducting or electronic properties of metallic. Carbon nanosheets were discovered in 1991 by S. Iijima. Due to the unique chemical and physical properties of carbon nanosheets(CNTS), there is a hopeful continuous growth of telecommunication. The length range of CNTS is less than 100 nm to 0.5 m. Carbon nanosheets(CNTS) classify into two types: single-walled carbon nanosheets and multiwalled carbon nanosheets.(1)
Single-walled carbon nanosheets(SWCTs): Single-walled carbon nanosheets consist of single graphene. It does not have any complex structure because of impurity. It bends easily [27].(2)
Multiwalled carbon nanosheets(MWCTs): Multiwalled carbon nanosheets consist of multigraphene. It has a complex structure due to its high purity. It cannot be bent easily [28, 29].Now, we construct the quadratic polynomial which best fits the curve. We will also find the difference betweenEexactT, EestimatedT, and EEexactT, EEestimatedT of TUC4C8 nanosheet 1 as shown in Figure 1.Figure 1
TUC4C8 nanosheet.Proposition 1.
LetT be the graph of TUC4C8 nanosheet, then the energy ET and the Estrada index EET of T are given below:(1)EN=−1.8×10−3m4n2+1.91×10−2m3n2−6.96×10−2m2n2+1.02×10−1m1n2−5.07×10−2n2+2.64×10−2nm4−2.985×10−1nm3+1.1526nm2+4.1125nm+2.748n−213×10−2m4+2.228×10−1m3−7.712×10−1m2+2.8762m−3.89×10−1,(2)EEN=−3×10−4m4n2+3.6×10−3m3n2−1.6×10−2m2n2+2.86×10−2mn2−1.6×10−2n2+1.6×10−3m4n−2.01×10−2m3n+8.74×10−2m2n+13.379mn+3.127n−2×10−3m4+2.5×10−2m3−1.081×10−1m2+3.2289m−1.02×10−1.The equations (1) and (2) characterize the energy EN and Estrada index EEN, respectively. We have established the inequality EexactN>EestimatedN, where Eexact and Eestimated show the exact and estimated value of energy. This inequality poses that the exact value Eexact is always greater than Eestimated. So, the error among Eexact and Eestimated is always positive.But on the other hand, the exact valueEEexactN is always smaller than EEestimatedN which describes the error between EEexact and EEestimated is negative.In Table1, m is an integer from 1 to 10 while n is fixed, and in Tables 2 and 3 quadratic equations are constructed.Table 1
The quadratic equation for theEN and EEG of TUC4C8 nanosheet.
m,nEnergy (E(N))Estrada index (EE(N))1,n−0.001n2+7.741n+1.9175−0.0001n2+16.5749n+3.04182,n−0.0011n2+13.6178n+3.72020.0012n2+30.0994n+6.009143,n−0.0012n2+19.5378n+5.5891−0.0013n2+43.6375n+9.12484,n0.0053n2+25.294n+7.583−0.004n2+57.1646n+12.1725,n−0.0182n2+31.313n+9.2495−0.0105n2+70.6945n+15.2156,n−0.1515n2+38.655n+9.625−0.0316n2+84.2794n+18.18787,n−0.5176n2+49.0138n+7.2347−0.0853n2+98.0099n+20.97648,n−1.2827n2+64.7168n+0.0926−0.1968n2+112.015n+23.41889,n−2.6562n2+88.725n−14.2985−0.3985n2+126.4621n+25.30510,n−4.8907n2+124.633n−38.947−0.73n2+141.557n+26.377Table 2
All aforementioned are derived from the coefficient of the curves given in Table1.
nEnergy (E(N))n2−18×10−3m4+191×10−2m3−696×10−2m2+102×10−1m−507×10−2n264×10−2m4−2985×10−1m3+1.1526m24.1125m+2.7481−213×10−2m4+2228×10−1m3−7712×10−1m2+2.8762m−389×10−1Table 3
All aforementioned equations are derived from the coefficient of the curves given in Table1.
nEstrada index EE(N)n2−3×10−4m4+3.6×10−3m3−1.6×10−2m2+2.86×10−2m−1.6×10−2n1.6×10−3m4−2.01×10−2m3+8.74×10−2m2+13.379m+3.1271−2×10−3m4+2.5×10−2m3−1.081×10−1m2+3.2289m−1.02×10−1Error among theEexact and Eestimated are shown in Table 4 and (Table 5).Table 4
TheEexactT and EestimatedT of TUC4C8 nanosheet.
m,nEexactTEestimatedTError(5, 1)40.601240.54430.0569(5, 2)71.802771.76460.0381(5, 3)103.5458103.34150.2043(5, 4)134.4458134.21030.2355(5, 5)165.8416165.35950.4821Table 5
TheEEexactT and EEestimatedT of the TUC4C8 nanosheet.
m,nEEexactT (N)EEestimatedTError(4, 1)69.332669.3403−0.0077(4, 2)126.4852126.516−0.0308(4, 3)183.6298183.689−0.0592(4, 4)240.7664240.861−0.0946(4, 5)298.895298.04−1


## 4. Energy and Estrada Index of Naphthalene

Naphthalene is a very important polycyclic aromatic hydrocarbon and it is useful in indoor and outdoor environments. It is actually a part of crude oil. The construction of naphthalene is based on two fused benzene rings that are connected with each other. Naphthalene is also used in mothballs which are widely used throughout the world. It is also used as an insect repellent product. Naphthalene is vibrantly volatile and insoluble in water, also the part of coal tar. In this section, we find the energy and Estrada index of naphthalene 2 which is given in Figure2.Figure 2
Naphthalene.Proposition 2.
LetN be the chemical graph of naphthalene. Then, the energy EN and Estrada index EEN of the chemical graph of naphthalene are given below.(3)EN=2×10−4m2n2−2×10−4mn2+1.9×10−3n2−1.2×10−3nm2+15.176nm−5.967×10−1n+1.8×10−3m2−9.097×10−1m−1.79×10−2,(4)EEN=−1×10−4m6n2+4.1×10−3m5n2−4.23×10−2m4n2+1.805×10−1m3n2−2.864×10−1m2n2+7.98×10−2mn2+5.27×10−2n2+6×10−4m6n−1.63×10−2m5n+1.59×10−1m4n−5.569×10−1m3n+1.969×10−1m2n+34.682mn−5.6672n−5.1×10−3m6+1.755×10−1m5−2.3788m4+15.91m3−54.385m2+83.077m−45.943.Equation (3) represents the energy and (4) represents the Estrada index, respectively. E(N) shows the energy of naphthalene and EE(N) shows the Estrada index of naphthalene. In the case of naphthalene, we observed this inequality EexactN>EestimatedN, where EexactN and Eestimated shows the exact and estimated value of energy, respectively.In the case of the Estrada index, we observed this inequalityEEexactN<EEestimatedN , where EEexactN and EEestimated reflects the exact and the estimated value of the Estrada index. The detailed analysis has been discussed critically.In Table6, m is an integer from 1 to 10 and n is fixed, and in Tables 7 and 8 quadratic equations are constructed.Table 6
The polynomial of order 2 for theEG and EEN of naphthalene.
m,nEnergy (E(N))Estrada index (EE(N))1,n0.0019n2 + 14.5781n − 0.9285−0.0117n2 + 28.7981n − 3.54942,n0.0023n2 + 29.7505n − 1.8301−0.0413n2 + 62.09n − 2.82023,n0.0031n2 + 44.9205n − 2.73080.085n2 + 94.4701n − 10.36124,n0.0043n2 + 60.0881n − 3.62790.03015n2 + 127.04n − 15.70545,n0.0059n2 + 75.2533n − 4.52140.06667n2 + 160.8653n − 19.4336,n0.0079n2 + 90.4161n − 5.41131.6043nn2 + 196.5316n − 24.96347,n0.0103n2 + 105.5765n − 6.29764.0707n2 + 234.1325n − 34.01928,n0.0131n2 + 120.7345n − 7.18039.6511n2 + 273.6896n − 45.76229,n0.0163n2 + 135.8901n − 8.059420.5835n2 + 316.0045n − 59.601410,n0.0199n2 + 151.0433n − 8.934939.7109n2 + 363.9428n − 81.673Table 7
The polynomial of order 2 derived from the coefficient of the curves given in Table6.
nEnergy (E(N))n22×10−4m2−2×10−4m+1.9×10−3n−1.2×10−3m2+15.176m−5.967×10−111.8×10−3m2−9.097×10−1m−1.79×10−2Table 8
The polynomial of order 2 derived from the coefficient of the curves given in Table6.
nEstrada index EE(N)n2−0.0001m6+0.0041m5−0.0423m4+0.1805m3−0.2864m2+0.0798m+0.0529n0.0006m6−0.0163m5+0.159m4−0.5569m3+0.1969m2+34.682m−5.66721−0.0051m6+0.1755m5−2.3788m4+15.91m3−54.385m2+83.077m−45.943Also, the error between theEexactN, EestimatedN, and EEexactN, EEestimatedN are shown in Tables 9 and 10.Table 9
TheEexactN and EestimatedN values of the naphthalene.
m,nEexactNEestimatedNError(2, 1)27.979827.92270.0571(2, 2)57.680157.63220.0479(2, 3)87.442187.32020.1219(2, 4)117.2087117.120.0887(2, 5)146.9799146.95140.0285(2, 6)176.7557176.7390.0167(2, 7)206.5361206.2480.0345(2, 8)236.3211236.2480.0731(2, 9)266.1107266.02060.0901(2, 10)295.9049295.82520.0797Table 10
TheEEexactN and EEestimatedN values of the Estrada indices of the naphthalene.
m,nEEexactNEEestimatedNError(3, 1)83.1125691.6297–8.51714(3, 2)177.7729186.3548–8.5819(3, 3)272.012281.2499–9.2379(3, 4)366.2511376.315–10.0639(3, 5)460.4902471.5501–11.0599(3, 6)554.7293566.9552–12.2259(3, 7)648.9684662.5303–13.5619(3, 8)743.2075758.2754–15.0679(3, 9)837.4466854.2309–16.7843(3, 10)932.1173950.2756–18.1583


## 5. Conclusion

In this paper, we compute the energyEexactN and Estrada index EEestimatedN of TUC4C8 nanosheet. Similarly, we compute the energy EexactN and Estrada index EEestimatedN of naphthalene. We have observed some inequalities: the exact value of the energy of TUC4C8 nanosheet and naphthalene are both greater than the estimated value of TUC4C8 nanosheet and naphthalene, respectively.(1).
EexactN>EestimatedN(2)
EexactT>EestimatedTThis concludes that the error among the abovementioned structures is always positive.Similarly, the exact value of the Estrada index ofTUC4C8 nanosheet and naphthalene are both smaller than the estimated value of TUC4C8 nanosheet and naphthalene, respectively.(1).
EEexactN<EEestimatedN(2)
EEexactT<EEestimatedTThis concludes that the error among the abovementioned structures is always negative.



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