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Theoretical study of energy, inertia and nullity of phenylene and anthracene

  • Zaheer Ahmad , Zeeshan Saleem Mufti , Muhammad Faisal Nadeem EMAIL logo , Hani Shaker and Hafiz Muhammad Afzal Siddiqui
Published/Copyright: May 13, 2021

Abstract

Energy of a molecule plays an important role in physics, chemistry and biology. In mathematics, the concept of energy is used in graph theory to help other subjects such as chemistry and physics. In graph theory, nullity is the number of zeros extracted from the characteristic polynomials obtained from the adjacency matrix, and inertia represents the positive and negative eigenvalues of the adjacency matrix. Energy is the sum of the absolute eigenvalues of its adjacency matrix. In this study, the inertia, nullity and signature of the aforementioned structures have been discussed.

1 Introduction

A molecular graph is a mathematical object defined as G = ( V , E ) , where V is the set of vertices also called atoms and E is the set of edges also called bonds. Usually in molecular graphs, hydrogen atoms are not considered. The graph in this study is finite, undirected and simple. The degree of the vertex v is the number of vertices adjacent to that vertex v . Adjacent vertices are those which have edges in between them. Adjacent edges are those that have common vertex in between them. Let A = [ a i j ] , i , j { 1 , 2 , 3 , .. . , q } be the adjacency matrix of the graph G , and A is the square matrix of order n . The eigenvalues λ 1 , λ 2 , λ 3 , .. . , λ q 1 , λ q of A are called the eigenvalues of the graph G . A is a symmetric matrix with the properties that its trace is zero, all of its eigenvalues are real and sum of all these eigenvalues is zero. The absolute sum of all the eigenvalues is called an energy of the graph G . Mathematically, it is denoted as E = i = 1 q λ i . The set of all eigenvalues is also called the spectrum of the graph G .

The eigenvalues play an important role in the field of mathematics, but these values are also very important in other fields such as chemistry, economics and many more. As far as our study concerns about eigenvalues, these values interpret in chemistry not only as the form of energy but also as different physicochemical properties of a chemical compound. We need to understand the relationship between mathematics and chemistry. The positive eigenvalues are linked with antibonding level, negative eigenvalues are linked with bonding levels and zero eigenvalues are linked with nonbonding level [13]. The number of positive eigenvalues p ( G ) represents the positive inertia index, while the number of negative eigenvalues n ( G ) represents negative inertia index; and nullity η ( G ) is the number of zeros that appear in the roots of characteristics polynomial. All the eigenvalues form a set called spectrum of G . The signature s ( G ) of the graph G is just a number that is generated by the difference in the number of positive and negative eigenvalues. Because nullity plays a vital role in the stability of molecules and if nullity is zero, this shows that the molecule is predicted to have a stable, closed-shell, electron configuration. If nullity is greater than zero, then the molecule is unstable, highly reactive, nonexistent and open shell. So it is observed that phenylene and anthracene are stable, closed-shell molecules because they are represented by their nullity which is equal to zero.

Phenylene belongs to the special class of conjugated hydrocarbons which plays an important role in the field of chemistry. The geometrical construction of phenylene is as follow: its shape is based on two mathematical objects, namely, hexagon and square. These objects are connected in such a way that a square is connected with two hexagons, which show that every two hexagons are totally separated (see Figure 1).

Figure 1 
               Phenylene.
Figure 1

Phenylene.

The molecular graph of anthracene consists of three fused benzene rings (see Figure 2). Actually, it is a part of coal tar which is a very useful component in daily life. Coal tar, which contains around 1.5% anthracene, remains a major source of this material. Its chemical formula is C 14 H 10 . It is used in the production of the different dyes.

Figure 2 
               Anthracene.
Figure 2

Anthracene.

Quantitative structure activity relationship (QSAR) is a regression and classification model is used in chemical and biological sciences and for many other purposes, but the most important one is to characterize the chemical structures of different compounds in terms of a single real number. This number represents the correlation between chemical structures of different compounds and the related chemical and biological activities or properties [4,5]. There are a lot of categorical QSAR models (categorical models are not correlation driven) but the topological index is the popular one because of its effectiveness [1,6].

For further study of energy, see refs. [79]. A graph spectrum-based invariant, put forward by Estrada, is defined as E E ( G ) = i = 1 n e λ i . Although invented in the year 2000, the Estrada index has already found a remarkable variety of applications in refs. [1016]. Initially it was used to quantify the degree of folding of long-chain molecules, especially protein. Malik and Farooq have found the energy and Estrada index of TUC 4 C 8 [ m , n ] nanotubes and also found some conjuncture on energy and Estrada index [17,18]. For more results on Estrada index, see ref. [19]. Ma et al. discussed the positive and negative inertia index of trees, unicyclic graphs and bicyclic graphs [20]. In ref. [21], Omidi studied the nullity of bipartite graphs.

In this study, we have observed the following inequalities: E x ( P ) > E s ( P ) , E E x ( P ) < E E s ( P ) , E x ( A ) > E s ( A ) and E E x ( A ) > E E s ( A ) , where E x ( P ) , E s ( P ) , E x ( A ) and E s ( A ) are the exact and estimated values of energy of phenylene and anthracene, respectively, and E E x ( P ) , E E s ( P ) , E E x ( A ) and E E s ( A ) are the exact and estimated values of Estrada index of phenylene and anthracene, respectively. In addition, we have also calculated the inertia, nullity and signature of phenylene and anthracene. The exact values are those values that we calculate from the graph by using softwares, and estimated values are those that we extract from the (polynomial) curves.

2 Method

In this section, we discuss the process of finding the energy and Estrada index of the phenylene and anthracene by using the computational methods. In the first step, we use HyperChem to draw the molecule of our chemical structures [22]. In the second step, we construct an adjacency matrix of the graph by using TopoCluj [23]. In the third step, we find the energy with the help of MATLAB which is represented in Table 1. In the fourth step, we find the suitable curve (polynomial) of degree two with the help of “cftoolbox” of MATLAB [24,25]. The results obtained from this data are displayed in Table 2. In Tables 3 and 4, we found the errors between the exact and the estimated values extracted from the curve for energy and Estrada index of phenylene. We also calculated the inertia index, nullity and signature of the phenylene and are displayed in Table 5. Similar process has been used for anthracene. In Tables 6 and 7, we found the polynomials by using the abovementioned techniques, and then in Tables 8 and 9, we calculated the errors of the exact and estimated values of the energy and Estrada index of anthracene. Finally, in Table 10, we displayed the inertia index, nullity and signature of anthracene.

Table 1

The quadratic curves for the energy and Estrada index of phenylene

( m , n ) Energy ( E ( P ) ) Estrada index ( E E ( P ) )
( 1 , n ) 0.0009 n 2 + 8.4872 n 0.4467 9 × 10 9 n 2 + 18.089 n 4.3925
( 2 , n ) 0.0027 n 2 + 17.354 n 1.0234 6 × 10 8 n 2 + 35.724 n 3.9426
( 3 , n ) 0.0001 n 2 + 26.226 n 1.4013 1 × 10 9 n 2 + 55.789 n 5.9133
( 4 , n ) 0.0011 n 2 + 35.143 n 1.9339 3 × 10 8 n 2 + 74.234 n 6.2703
( 5 , n ) 0.0004 n 2 + 44.12 n 2.6087 3 × 10 8 n 2 + 93.085 n 7.0307
( 6 , n ) 0.0008 n 2 + 52.982 n 3.0532 4 × 10 8 n 2 + 111.93 n 7.7911
( 7 , n ) 0.0011 n 2 + 61.888 n 3.5766 4 × 10 8 n 2 + 130.78 n 8.5516
( 8 , n ) 0.0014 n 2 + 70.793 n 4.1001 5 × 10 8 n 2 + 149.63 n 9.312
( 9 , n ) 0.0016 n 2 + 79.698 n 4.6236 5 × 10 8 n 2 + 168.48 n 10.072
( 10 , n ) 0.0019 n 2 + 88.604 n 5.147 6 × 10 8 n 2 + 187.34 n 10.833
Table 2

The quadratic curves extracted from the coefficient of the equations given in Table 1

Energy ( E ( P ) ) Estrada index  ( E E ( P ) )
n 2 6 × 10 6 m 2 0.0002 m + 0.0007 6 × 10 11 m 2 + 6 × 10 9 m + 6 × 10 9
n 0.0005 m 2 + 8.8999 m 0.4393 0.0017 m 2 + 18.831 m 1.1281
1 0.0007 m 2 0.5161 m + 0.0729 0.0015 m 2 0.7436 m 3.2623
Table 3

The exact values E x ( P ) and estimated values E s ( P ) of the energies of the Phenylene

( m , n ) E x ( P ) E s ( P ) Error
( 10 , 1 ) 84.51 84.21 0.30
( 10 , 2 ) 172.99 172.69 0.30
( 10 , 3 ) 261.40 261.17 0.23
( 10 , 4 ) 349.96 349.66 0.30
( 10 , 5 ) 438.61 438.15 0.46
( 10 , 6 ) 527.16 526.66 0.50
( 10 , 7 ) 615.43 615.17 0.26
( 10 , 8 ) 704.11 703.69 0.42
( 10 , 9 ) 792.63 792.22 0.41
( 10 , 10 ) 881.11 880.74 0.37
Table 4

The exact values E E x ( P ) and estimated values E E s ( P ) of the Estrada index of the phenylene

( m , n ) E E x ( P ) E E s ( P ) Error
( 10 , 1 ) 176.5041 176.5141 0.0099
( 10 , 2 ) 363.8355 363.8555 0.0200
( 10 , 3 ) 551.1702 551.2074 0.0372
( 10 , 4 ) 738.5094 738.5593 0.0499
( 10 , 5 ) 925.8424 925.9112 0.0687
( 10 , 6 ) 1113.1775 1113.2631 0.0851
( 10 , 7 ) 1300.5126 1300.6150 0.1023
( 10 , 8 ) 1484.8476 1487.9669 0.1189
( 10 , 9 ) 1675.1827 1675.3188 0.1358
( 10 , 10 ) 1862.5178 1862.6707 0.1527
Table 5

The inertia, nullity and signature of phenylene

P = Phenylene p (P) n (P) η (P) s (P)
(8, 1) 24 24 0 0
(8, 2) 48 48 0 0
(8, 3) 72 72 0 0
(8, 4) 96 96 0 0
(8, 5) 120 120 0 0
(8, 6) 144 144 0 0
(8, 7) 168 168 0 0
(8, 8) 192 192 0 0
(8, 9) 216 216 0 0
(8, 10) 240 240 0 0
(8, 11) 264 264 0 0
(8, 12) 288 288 0 0
Table 6

The quadratic curves suitable for the Energy and Estrada index of anthracene

( m , n ) Energy ( E ( A ) ) Estrada index ( E E ( A ) )
( m , 1 ) 7 × 10 6 n 2 + 42.356 n 3.2224 3 × 10 14 n 2 + 135.29 n + 8.3861
( m , 2 ) 3 × 10 6 n 2 + 63.382 n 4.1488 1 × 10 13 n 2 + 229.68 n 32.84
( m , 3 ) 9 × 10 6 n 2 + 85.212 n 6.1953 1 × 10 13 n 2 + 324.07 n 74.066
( m , 4 ) 3 × 10 6 n 2 + 106.53 n 7.5104 3 × 10 13 n 2 + 418.46 n 115.29
( m , 5 ) 4 × 10 6 n 2 + 127.94 n 8.9824 2 × 10 13 n 2 + 512.85 n 156.52
( m , 6 ) 4 × 10 6 n 2 + 149.38 n 10.476 1 × 10 13 n 2 + 607.23 n 197.74
( m , 7 ) 3 × 10 6 n 2 + 170.82 n 11.965 6 × 10 14 n 2 + 701.62 n 238.97
( m , 8 ) 3 × 10 6 n 2 + 192.25 n 13.453 5 × 10 13 n 2 + 796.01 n 280.2
( m , 9 ) 2 × 10 6 n 2 + 213.68 n 14.941 2 × 10 13 n 2 + 890.4 n 321.42
( m , 10 ) 2 × 10 6 n 2 + 235.11 n 16.429 5 × 10 13 n 2 + 984.79 n 362.65
Table 7

The quadratic curves extracted from the coefficient of the equations in Table 6

Energy ( E ( A ) ) Estrada index ( E E ( A ) )
n 2 2 × 10 8 m 2 2 × 10 7 m + 5 × 10 6 3 × 10 15 m 2 + 2 × 10 14 m 1 × 10 13
n 2 × 10 5 m 2 + 21.432 m + 20.789 3 × 10 14 m 2 + 94.388 m + 40.903
1 4 × 10 5 m 2 1.4874 m 1.5503 7 × 10 5 m 2 41.225 m + 49.611
Table 8

The exact values E x ( A ) and estimated values E s ( A ) of the energy of the anthracene

( m , n ) E x ( A ) E s ( A ) Error
( 10 , 1 ) 218.7247 218.6827 0.0420
( 10 , 2 ) 453.8533 453.7937 0.0596
( 10 , 3 ) 688.9118 688.9047 0.0071
( 10 , 4 ) 924.0977 924.0157 0.0820
( 10 , 5 ) 1159.1496 1159.1268 0.0229
( 10 , 6 ) 1394.2470 1394.2377 0.0093
( 10 , 7 ) 1629.3590 1629.3487 0.0103
( 10 , 8 ) 1864.4720 1864.4597 0.0123
( 10 , 9 ) 2099.5850 2099.5707 0.0143
( 10 , 10 ) 2334.6970 2334.6818 0.0152
Table 9

The exact values E E x ( A ) and estimated values E E s ( A ) of the Estrada index of the anthracene

( m , n ) E E x ( A ) E E s ( A ) Error
( 10 , 1 ) 622.1380 622.1370 0.0010
( 10 , 2 ) 1606.9220 1606.9200 0.0020
( 10 , 3 ) 2591.7070 2591.7030 0.0040
( 10 , 4 ) 3576.4930 3576.4860 0.0070
( 10 , 5 ) 4561.2790 4561.2690 0.0100
( 10 , 6 ) 5546.0650 5546.0520 0.0130
( 10 , 7 ) 6530.8500 6530.8350 0.0150
( 10 , 8 ) 7515.6360 7515.6180 0.0180
( 10 , 9 ) 8500.4220 8500.4010 0.0210
( 10 , 10 ) 9485.2070 9485.1840 0.0230
Table 10

The inertia, nullity and signature of anthracene

G = Anthracene p (G) n (G) η (G) s (G)
(8, 1) 35 35 0 0
(8, 2) 70 70 0 0
(8, 3) 105 105 0 0
(8, 4) 140 140 0 0
(8, 5) 175 175 0 0
(8, 6) 210 210 0 0
(8, 7) 280 280 0 0
(8, 8) 315 315 0 0
(8, 9) 350 350 0 0
(8, 10) 385 385 0 0
(8, 11) 420 420 0 0
(8, 12) 455 455 0 0

2.1 Results of phenylene

In this section, we will study the energy, Estrada index, nullity, inertia and signature of phenylene. We will also show the comparison between exact and estimated values of energy and Estrada index of phenylene.

Proposition 2.1

Let P be the graph of (m, n)-phenylene, then the energy and Estrada index of P are given by

(1) E ( P ) = 6 × 10 6 m 2 n 2 0.0002 n 2 m + 0.0007 n 2 + 0.0005 n m 2 + 8.8999 n m 0.4393 n 0.0007 m 2 0.5161 m + 0.0729

(2) E E ( P ) = 6 × 10 11 m 2 n 2 + 6 × 10 9 n 2 m + 6 × 10 9 n 2 + 0.0017 n m 2 + 18.831 n m 1.1281 n 0.0015 m 2 0.7436 m 3.2623

The two curves that are representing the energy and Estrada index of the graph are given by equations (1) and (2), respectively. We observed that the exact value of the energy of phenylene is always greater than the estimated values of the energy of phenylene, i.e., E x ( P ) > E s ( P ) , where E x represents the exact value of energy of phenylene and E s represents the estimated value of energy of phenylene. Also error is always positive between the exact and estimated values of energy of phenylene, i.e., Error > 0 .

Similarly, we have observed that the exact value of Estrada index of phenylene is always less than the estimated values of Estrada index of phenylene, i.e., E E x ( P ) < E E s ( P ) , where E E x represents the exact value of Estrada index and E E s represents the estimated value of Estrada index. As shown in Table 1, for every curve, the value of m and n ranges from 1 to 10.

Also the errors between the exact values and the estimated values of energy and Estrada index are summarized in Tables 3 and 4.

2.2 Results of anthracene

In this section, we will study the energy, Estrada index, nullity, inertia and signature of anthracene. We will also show the comparison between the exact and estimated values of energy and Estrada index of anthracene.

Proposition 2.2

Let A be the graph of (m, n)-anthracene, then the energy and Estrada index of A are given by

(3) E ( A ) = 2 × 10 8 m 2 n 2 2 × 10 7 n 2 m + 5 × 10 6 n 2 + 2 × 10 5 n m 2 + 21.432 n m + 20.789 n 4 × 10 5 m 2 1.4874 m 1.5503

(4) E E ( A ) = 3 × 10 15 m 2 n 2 + 2 × 10 14 n 2 m 1 × 10 13 n 2 + 3 × 10 14 n m 2 + 94.388 n m + 40.903 n 7 × 10 15 m 2 41.225 m + 49.611 .

The errors of energy and Estrada index are as given in Tables 8 and 9.

We observed that exact value of the energy of anthracene is always greater than the estimated values of the energy of anthracene, i.e., E x ( A ) > E s ( A ) . Error is always positive between the exact and estimated values of the energy of anthracene, i.e., Error > 0 . Similarly, we have observed that the exact value of Estrada index of anthracene is always greater than the estimated values of Estrada index of anthracene, i.e., E E x ( A ) > E E s ( A ) .

3 Conclusion

We studied Estrada index, energy, inertia, nullity and signature of phenylene and anthracene. The following inequalities E x ( P ) > E s ( P ) , E E x ( P ) > E E s ( P ) , E x ( A ) > E s ( A ) and E E x ( A ) > E E s ( A ) have been observed between the exact and estimated values of the energy of phenylene and anthracene.

Also, since the nullity of phenylene and anthracene is zero, the molecule of these structures are stable and closed shell. At the end, we give a graphical representation of these parameters (Figures 36).

Figure 3 
            Energy of phenylene.
Figure 3

Energy of phenylene.

Figure 4 
            Energy of anthracene.
Figure 4

Energy of anthracene.

Figure 5 
            Estrada index of phenylene.
Figure 5

Estrada index of phenylene.

Figure 6 
            Estrada index of anthracene.
Figure 6

Estrada index of anthracene.

  1. Author contributions: Z. A. analyzed the data curation. Z. S. F. contributed to visualization, investigation and wrote the initial draft of the study. M. F. N. contributed to conceptualization, visualization and methodology. H. S. contributed to designing the experiments, validation. H. M. A. S. was in charge of formal analyzing of experiments, resources and some computations. All authors read and approved the final version of the study.

  2. Funding information: The authors received no financial support for the research, authorship, and/or publication of this article.

  3. Conflict of interest: The authors have no conflicts of interest.

  4. Ethical approval: The conducted research is not related to either human or animal use.

  5. Data availability statement: All data generated or analyzed during this study are included in this published article.

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Received: 2018-12-29
Revised: 2019-12-01
Accepted: 2020-08-13
Published Online: 2021-05-13

© 2021 Zaheer Ahmad et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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