ORIGINAL RESEARCH

Intramolecular hydrogen bonding in chemoselective synthesized2-substituted pyrrole stable phosphorus ylide: GIAO, AIM,and NBO approaches

Mehdi Shahraki • Sayyed Mostafa Habibi-Khorassani •

Ali Ebrahimi • Malektaher Maghsoodlou •

Younes Ghalandarzehi

Received: 3 August 2012 / Accepted: 21 August 2012 / Published online: 2 September 2012

� Springer Science+Business Media, LLC 2012

Abstract The chemoselectivity of geometrically ylide

compounds is often hard to assign from experimental tech-

niques, particular system with intramolecular hydrogen

bonding (IHB) are even more challenging. Herein, theoretical

calculations were performed to investigate whether theoreti-

cal results would provide consistent evidence for the exis-

tence of IHB to confirm experimental data and to evaluate

strength of the N–H���O IHB from geometrical synthesized

2-substituted pyrrole stable phosphorus ylide (dimethyl

2-(1H-pyrrol-2-yl)-3-(triphenylphosphoranylidene) butane-

dioate in a single chemoselective compound. Topological

parameters at the bond critical points (BCP) of intramolecular

hydrogen bonds from Bader’s atoms in molecules (AIM)

theory and Winhold’s natural bond orbital (NBO) calcula-

tions were analyzed at the B3LYP/6-311??g** level in

details. A series of gage-including atomic orbital chemical

shift (GIAO c.s.) calculations at the HF and DFT levels of

theory were carried out to assign the 1H NMR chemical shifts.

The best prediction of the experimental 1H NMR values was

obtained at the mPW1PW91 levels using the 6-31G** basis

set. Theoretical results, in agreement with the experimental

data, were confirmed the N–H���O IHB was caused the

deshielding of the proton to lower field. The barriers in

P–C C–O6 double bond and CC CN single bond rotation

were theoretically estimated in detailed and the AIM and

NPA approaches were confirmed the loss of charge of the

hydrogen atom involving in intramolecular N–H���O hydro-

gen bonding. The geometrical and topological parameters

from AIM and NBO analyses were indicated the medium

N–H���O IHB.

Keywords Ylide � Intramolecular hydrogen bonding �NMR GIAO c.s � AIM � NBO � Chemoselective

Introduction

Nitrogen-containing heterocyclic compounds such as pyr-

role and its derivatives are important in organic chemistry

since their structures can be found in many natural or

therapeutic compounds and are widely used in medicine as

antibiotic, in biological activities, and organic polymers

[1–3]. The synthesis and structural studies of organophos-

phorus compounds have been one of the great interest

[4–7] and the main research focus points of our laboratory

in last decade [8–15].

Establishing the chemoselective of geometrically ylide

compounds can be particularly challenging, and it may be

necessary to resort to the time-consuming synthesis of all

potential isomer to find which of these match the real

product [16]. In continuation of our research works on

development of enaminoester and phosphorus ylide com-

pounds, we have published two papers on the synthesis and

structural studies of pyrrole stable phosphorus ylide

[17, 18]. In these works, we have prepared the novel pyr-

role-containing phosphorous ylide using a one-pot reaction

between triphenylphosphine and dialkyl acetylendicarb-

oxylates in the presence of pyrrole in a mixture of aqueous-

organic media (water–acetone 30:70) and dry ethylacetate

as solvent (Fig. 1). According to the 1H and 13C NMR data,

Electronic supplementary material The online version of thisarticle (doi:10.1007/s11224-012-0114-z) contains supplementarymaterial, which is available to authorized users.

M. Shahraki � S. M. Habibi-Khorassani (&) � A. Ebrahimi �M. Maghsoodlou � Y. Ghalandarzehi

Department of Chemistry, Faculty of Science, University of

Sistan and Baluchestan, P.O. Box 98135-674, Zahedan, Iran

e-mail: smhabibikhorassani@yahoo.com

123

Struct Chem (2013) 24:623–635

DOI 10.1007/s11224-012-0114-z

two geometrical isomers of N-substituted pyrrole stable

phosphorus ylide were observed in dry ethylacetate as

solvent. Theoretical studies on the two 2-(Z) and

2-(E) isomers were carried out using NBO analysis and

AIM theory along with 1H NMR chemical shifts investi-

gations. The obtained results were confirmed existence of

the two 2-(Z) and 2-(E) isomers in dry ethylacetate at

ambient temperature [19].

The chosen synthetic route that be able to selectivity

deliver a single compound by choosing suitable starting

materials, reagents, solvents, reaction conditions, and cat-

alysts is an important principle of organic synthesis, green

chemistry, synthesis of drugs, etc. [20]. An analogous to

dry ethylacetate, water–acetone media enforced chemose-

lective synthesis of fixed product 1-(Z) (2-substituted pyr-

role stable phosphorus ylide) as a single isomer with Z-

orientation of carbon–carbon double bond at ambient

temperature [17, 18]. The 1H, 13C NMR, IR spectroscopy,

and mass spectral data were shown high yield and good

selectivity by choice of aqueous solvent. The evidence of

existence intramolecular hydrogen bonding (IHB) was

directly obtained from the experimental 1H NMR data [18].

The IHB is capable of being responsible for the

molecular geometry, as well as for the stability of certain

predominant isomer. As we have revealed, the adequate

theoretical studies in this area are lacking. The main

objective of this study was to investigate whether theoret-

ical results would provide clear evidence for the existence

of IHB to confirm the experimental data and evaluate

strength of the intramolecular N–H���O hydrogen bonding

to fix product 1-(Z) as a single chemoselective compound.

Quantum theory based upon calculation of NMR

parameters is now a mature approach that can significantly

widen the interpretative and analytical power of one of the

most important spectroscopic techniques [21–24]. The

gage-including atomic orbital (GIAO) method is one of the

most successful approaches for calculations of chemical

shifts of medium–large organic compounds [25–27].

Herein, a series of calculations at the HF and density

functional theory (DFT) (B3LYP, mPW1PBE, and

mPW1PW9127) levels in the same geometry optimization

(HF/6-31G**) [28] on the product 1-(Z) shown in Fig. 1,

which its experimental 1H chemical shifts have been

reported in the literature [18]. Based on this study, we have

suggested that, among the methods taken into consider-

ation, the best prediction of the experimental 1H NMR

values was obtained at the mPW1PW91 levels using the

6–31G** basis set.

Rotational barriers are significant in determining the

inherent ‘‘stiffness’’ of macromolecular chains and hence

are used to assess isomerism. Intra and intermolecular

hydrogen bonding, and steric interactions serve to change

the rotational barrier sufficiently [29]. Rotational about the

bond has been generally studied by dynamic 1H NMR

spectroscopy and theoretical methods [30–33]. In this

article, towards of our dynamic studies [13, 30], the energy

barrier in P–C C–O6 double bond and CC CN single

bond rotation were theoretically examined in detail.

A frequent object of quantum chemical studies is the

determination of the electronic configuration and net

charge associated with each atom in a polyatomic mole-

cule. Information concerning atomic charge distributions is

important in rendering a chemical interpretation of the

wave function, leading to a meaningful interpretation and

an ability to draw analogies between different chemical

phenomena. The calculation of these properties poses the

problem of how the calculated electron density should be

‘‘distributed within a molecule’’ [34, 35].

The atomic charges schemes were employed and com-

pared in this study include the AIM, NBO, Mulliken, and

CHELPG methods [36, 37]. We have resorted to DFT to

perform all computations. It was understood that DFT is

one of the most flexible and reliable quantum mechanical

techniques, appropriate for the relative large size com-

pounds studied here, since simpler methods unfortunately

+PPh3

1-(Z)

2-(Z) 2-(E)

+N

H CO2CH3

H

Ph3P O

CO2CH3

NCO2CH3

H

Ph3P OCH3

O

N

4 4

NH O OCH3

H3CO2CH PPh3

CH3OOC COOCH3

water-acetone

dry ethyl acetate

58 6

(a)

(b)

Fig. 1 The reaction between triphenylphosphine, dimethyl acetylendicarboxylate, and pyrrole for generation of stable phosphorus ylides in

different solvents. (a) A single isomer in a mixture of water–acetone, (b) the two 2-(Z) and 2-(E)-isomers in dry ethylacetate

624 Struct Chem (2013) 24:623–635

123

fail on accuracy, while higher accurate schemes demand

for time-consuming computations [38–40].

The atoms in molecules (AIM) theory of Bader was

also applied here to study the properties of the bond

critical point and to analyses dependencies between

topological, energetic and geometrical parameters of the

IHB [41, 42].

Computational details

The Gaussian 03 program package [43] was used to per-

form all theoretical calculations on the molecular structure.

In order to calculate accurately 1H chemical shifts avoiding

time-consuming approaches, HF-optimized structure was

employed as input for the GIAO 1H chemical shift calcu-

lations at the HF and DFT (B3LYP, mPW1PBE, and

mPW1PW9127) levels of theory using 6-31G** basis set.

The consistency and efficiency of the considered combi-

nations of geometry optimization and GIAO 1H NMR

calculations were thoroughly checked by the analysis of

statistical parameters concerning computed and obtained

experimental 1H NMR chemical shifts values from a

Bruker DRX-500 Avence instrument with CDCl3 as sol-

vent at 500.1 MHz.

The AIM theory of Bader was applied to find the critical

points [41, 42, 44, 45] and to analyze them in terms of

electron densities q(BCP) and their Laplacian r2q(BCP). The

AIM calculations were carried out using the AIM2000

package [46].

Two kinds of rotational barrier were considered by

scanning method at the HF/6-31??g** level. Structures of

ground and transition-states (TS) found for the product

1-(Z) were examined at the B3LYP/6-31??g** level. The

Mulliken, NBO, AIM, and CHELPG methods have been

calculated at the B3LYP/6-31??g** level [34, 47–50].

Results and discussion

Comparison of chemical shift calculations

Because of the huge size of the product 1-(Z) and in order

to find a sound and fast method for analyzing medium–

large organic molecules, and hence to compare accurately

the experimental and calculated 1H chemical shifts avoid-

ing time-consuming approaches, the HF-optimized struc-

ture of the product 1-(Z) was performed input for the NMR

calculations using the 6-31G** basis set. The single-point1H (GIAO) c.s. calculations were carried out at the HF,

B3LYP, mPW1PBE, and mPW1PW91 levels using the

6-31G** basis set [28]. Table 1 shows the comparison

between the experimental and calculated 1H chemical

shifts for the product 1-(Z) (see atomic numbering in

Fig. 2). The dexp values were relevant to the literature data

[18]. The dcalcd value of a given H atoms, Hi, were obtained

by subtracting 1H isotropic magnetic shielding parameter

values, r, (calculated by the single-point GIAO methods)

from the average 1Hd value, hri, of the tetramethyl silane

(TMS) carbon atoms: dcalcd = |hriTMS - rHi|. The empir-

ically scaled (dscaled) 1H c.s. values in Table 2 were taken

into consideration because these were shown previously

that chemical shifts calculated at lower theory levels could

be scaled using experimental information to achieve results

close to those obtained at higher theory levels, and hence to

analyze also this possibility [51, 52]. The intercept (b) and

Table 1 Calculated and experimental 1H NMR chemical shifts (ppm)

for product 1-(Z)

H-numbera dexpb dcalcd

c dcalcdd dcalcd

e dcalcdf

H36 7.72 10.22 10.46 10.04 10.04

H58 10.24 9.73 10.08 9.60 9.31

H23 7.72 8.45 8.63 8.26 8.23

H39 7.60 8.37 8.58 8.14 8.02

H40 7.43 8.16 8.40 7.95 7.94

H25 7.72 8.13 8.31 7.94 7.94

H45 7.72 8.09 8.17 7.89 7.90

H51 7.43 8.09 8.17 7.88 7.87

H49 7.60 8.00 8.16 7.81 7.77

H27 7.60 7.98 8.16 7.78 7.63

H29 7.43 7.97 8.13 7.76 7.62

H50 7.60 7.95 8.07 7.74 7.59

H38 7.60 7.92 8.06 7.72 7.58

H47 7.72 7.89 8.06 7.71 7.51

H28 7.60 7.89 7.94 7.68 7.51

H34 7.72 7.70 7.75 7.51 7.40

H60 6.70 6.97 7.37 6.79 6.85

H59 5.95 6.33 6.69 6.15 5.95

H56 5.25 5.67 5.98 5.49 5.23

H17 3.14 4.11 4.24 4.07 3.70

H18 3.14 3.95 4.13 3.91 3.50

H2 3.55 3.85 4.00 3.79 3.35

H12 3.67 3.80 3.86 3.73 3.32

H16 3.14 3.66 3.83 3.57 3.09

H14 3.67 3.54 3.52 3.52 3.06

H13 3.67 3.09 3.14 3.01 2.69

a Hydrogen numbers from Fig. 2b Run at 500.1 MHZ, on a Bruker DRX-500 Avence spectrometer

[18]c GIAO mPW1PW91/6-31G** level theoryd GIAO mPW1PBE/6-31G** level theorye GIAO B3LYP/6-31G** level theoryf GIAO HF/6-31G** level theory

Struct Chem (2013) 24:623–635 625

123

HN

C

H

H

H

H

CO

C

HCO

H

H

PPh3

O OC H

H

H

27

22

58

23

2

12

13

14

17

16

18

P

H

HH

H H

H

H

H

H

H

H

H

H

H

H

36

39

40

38

3423

27

2929

25

45 49

515047

6

9

32

Fig. 2 Hydrogen numbering of

2-substituted pyrrole stable

phosphorus ylide 1 (dimethyl

2-(1H-pyrrol-2-yl)-3-

(triphenylphosphoranylidene)

butanedioate) used in this study

Table 2 dscaleda and eb found for the different combinations of single-point GIAO c.s 1H NMR calculations (1H GIAO c.s. calculations//HF/6-

31G**)

Atoms dscaledc dscaled

d dscalede dscaled

f ec ed ee ef

36-H 9.75 10.17 9.81 9.36 2.50 2.74 2.32 2.32

58-H 9.27 9.79 9.37 8.66 0.51 0.16 0.64 0.93

23-H 8.00 8.31 8.01 7.64 0.73 0.91 0.54 0.51

39-H 7.92 8.26 7.89 7.44 0.77 0.98 0.54 0.42

40-H 7.71 8.07 7.70 7.36 0.73 0.97 0.52 0.51

25-H 7.68 7.98 7.69 7.36 0.41 0.59 0.22 0.22

45-H 7.64 7.84 7.63 7.32 0.37 0.45 0.17 0.18

51-H 7.64 7.84 7.62 7.30 0.66 0.74 0.45 0.44

49-H 7.56 7.83 7.55 7.20 0.40 0.56 0.21 0.17

27-H 7.54 7.83 7.52 7.06 0.38 0.56 0.18 0.03

29-H 7.53 7.79 7.50 7.05 0.54 0.70 0.33 0.19

50-H 7.51 7.74 7.48 7.02 0.35 0.47 0.14 0.01

38-H 7.47 7.73 7.46 7.01 0.32 0.46 0.12 0.02

47-H 7.45 7.72 7.45 6.94 0.17 0.34 0.01 0.21

28-H 7.45 7.61 7.42 6.94 0.29 0.34 0.08 0.09

34-H 7.26 7.41 7.25 6.85 0.02 0.03 0.21 0.32

60-H 6.54 7.02 6.52 6.32 0.27 0.67 0.09 0.15

59-H 5.90 6.33 5.87 5.46 0.38 0.74 0.20 0.00

56-H 5.26 5.60 5.20 4.77 0.42 0.73 0.24 0.02

17-H 3.71 3.83 3.76 3.31 0.97 1.10 0.93 0.56

18-H 3.55 3.71 3.59 3.12 0.81 0.99 0.77 0.36

2-H 3.45 3.58 3.47 2.98 0.29 0.45 0.24 0.20

12-H 3.41 3.44 3.41 2.95 0.13 0.19 0.06 0.35

16-H 3.27 3.41 3.25 2.73 0.52 0.69 0.43 0.05

14-H 3.15 3.10 3.20 2.70 0.13 0.15 0.15 0.61

13-H 2.70 2.71 2.68 2.35 0.58 0.53 0.66 0.98

a dscaled = (dcalcd - b)/a, a (slope) and b (intercept) from linear fitb e = |dexp - dscaled|c GIAO mPW1PW91/6-31G** level theoryd GIAO mPW1PBE/6-31G** level theorye GIAO B3LYP/6-31G** level theoryf GIAO HF/6-31G** level theory

626 Struct Chem (2013) 24:623–635

123

slope (a) were also determined according to linear fit the

experimental and calculated chemical shifts correlation

(see footnote in Table 2; Fig. 3).

Figure 3 shows correlation between experimental and

calculated chemical shifts in the same geometry optimi-

zations of the structure utilized as input in the GIAO sin-

gle-point 1H c.s. calculations. The mPW1PW91/6–31G**

approaches has proved to be the most efficient in predicting

the 1H chemical shifts of the product 1-(Z) taken into

consideration, displaying both the lowest mean absolute

error (MAE) parameters and the highest R2 coefficients,

also these latter chemical shift values are very close to the

experimental one (See Fig. 4).

Chemical shift of the H58 (d = 9.73 ppm, Tables 1 and

2) was deshielded to lower field and rationalized it as an

effect of IHB between the carbonyl oxygen atom (O6) and

the amine hydrogen (N–H58���O6, see Figs. 1, 2). In Fig. 5

changes in calculated chemical shifts at the mPW1PW91/

6–31G** level of theory as a function of the CC CNdihedral angle changes were estimated by comparing the

chemical shifts of the Z-isomer with IHB and the rotamers,

in which the hydroxyl group was at different angles to the

N–H58 bond, that should prevent the IHB. The obtained

results from Fig. 5 show a decrease of the chemical shifts is

consistent with an increase of the lengthening of N���O6 and

decrease in the IHB strength.

AIM analysis

The values of the charge density and its Laplacian at these

critical points give useful information regarding the

strength of the H-bonds [53]. Most important geometrical

and topological parameters are reported in Table 3. A

negative total energy density at the BCP reflects a domi-

nance of potential energy density, which is the conse-

quence of accumulated stabilizing electronic charge [54].

Fig. 3 Experimental and calculated 1H NMR chemical shifts corre-

lation and standard deviation (R2) for the product 1-(Z). a GIAO

mPW1PW91/6-31G** level theory, b GIAO mPW1PBE/6-31G**

level theory, c GIAOB3LYP/6-31G** level theory, and d GIAO HF/

6-31G** level theory

Fig. 4 Mean absolute error (MAE): R|dexp - dscaled|/n; (number of

compared chemical shifts), for the 1H GIAO c.s. single-point

calculations

Struct Chem (2013) 24:623–635 627

123

The values of q(H58���O6) and r2q(H58���O6) are 0.0210 and

0.0625 e/au5, respectively. With respect to the negative

amount of the Hamiltonian H(H58���O6), -0.0164 au. (See

Table 3), these intramolecular HBs showr2q(BCP) [ 0 and

HBCP \ 0, which according to classification of Rozas et al.

[55] was medium-strength hydrogen bond. Inspection of

the r2q(H36���O9) (0.0280 e/au5), q(H36���O9) (0.0084 e/au3),

and H(H36���O9)(-0.0054 hartrees/au3) reveals that the

intramolecular C32–H36���O9 hydrogen bonding was weak.

NBO analysis

The strengths of delocalization interactions, E(2), were

estimated by second order perturbation theory [34, 50]. In

Table 4, the NBO occupation numbers for the r*(N–H58)

and r*(C32–H58) antibonds, the oxygen’s lone pairs, nO6

and nO9, are represented in Fig. 2 and their respective

orbital energies, e, are reported. Furthermore, some of

significant donor–acceptor interactions and their second

order perturbation stabilization energies E(2) which were

calculated at the B3LYP/6-311??g** level are given in

Table 4. As illustrated in Table 4, the results of NBO

analysis show that in product 1-(Z), lone pairs of oxygen

atom (O6) participate as donor and the r*(N–H58) antibond

as acceptor in conventional intramolecular charge transfer

interaction. The sum E(2) terms corresponding to these

interactions can be considered to be total charge transfer

energy, ET(2), it was found to be 6.97 kcal/mol. Besides, the

Table 3 Geometrical (in A�) and electron density (q(BCP), e/au3), Laplacian (r2q(BCP), e/au5), and energy density (HBCP, hartrees/au3) calcu-

lated at the bond critical points of the product 1-(Z) at the B3LYP/6-31??g** level

H���O N���O qH���O r2q(H���O) G(rBCP) V(rBCP) HBCP

N–H58���O6 2.0671 2.8602 0.0210 0.0625 -0.0164 0.0001 -0.0164

H���O C���O qH���O r2q(H���O) G(rBCP) V(rBCP) HBCP

C32–H36���O9 2.5459 3.4867 0.0084 0.0280 0.0062 -0.0008 -0.0054

Table 4 Occupation numbers (electrons) and corresponding orbital

energies (e in unit au) of the X–H (X = N or C) bonds of the acceptor

and the lone pairs of the donor atom (nO), and their second order

perturbation stabilization energies E(2)(kcal/mol)

N–

H58���O6

C32–

H36���O9

ON(n1O6) 1.9697 ON(n2O9) 1.8498

e(n1O6) -0.6777 e(n2O9) -0.2475

ON(n2O6) 1.8465 E(2)(n2O9 ? r*C32–

H36)

0.87

e(n2O6) -0.2343 ON(r*C32–H36) 0.0161

ON(n3O6) 1.5945 e(r*C32–H36)

e(n3O6) -0.2274

E(2)(n1O6 ? r*N–

H58)

2.23

E(2)(n2O6 ? r*N–

H58)

1.32

E(2)(n3O6 ? r*N–

H58)

3.42

ON(r*N–H58) 0.0279

e(r*N–H58) 0.4819

Fig. 5 Changes in chemical

shifts obtained at the

mPW1PW91/6–31G** level of

theory as a function of the

CC CN dihedral angle

changes for the ground and

transition state points (see

Fig. 9)

628 Struct Chem (2013) 24:623–635

123

occupation number of this antibond being the around

0.0279e, whereas the intramolecular charge transfer inter-

action, n2O9 ? r*C32–H36 (E(2) = 0.87 kcal/mol and

ON(r*C32–H36) = 0.0161e) has been revealed weak IHB.

Comparison of charge distribution in hydrogen atoms

The usefulness of atomic charges as parameters for the

calculation of electrostatic interactions in a variety of

molecular mechanics simulation packages is certainly one

important area of application. Partial atomic charges serve

a different, but even more important, purpose in the qual-

itative rationalization of organic and inorganic reactivity

[36]. In the subsections below, the distribution analysis

obtained from the AIM, NPA, CHELPG, and Mulliken

methods for the hydrogen atoms is compared [34, 56–60].

The results from AIM and NPA analysis are summarized in

Table 5 and for making a better comparison; these are

presented as graphical form in Fig. 6. (The comparative

results from CHELPG and Mulliken analysis are given in

the supporting information.)

The partial charges obtained using the AIM scheme was

shown a significantly increment in H58 (qH58 =?0.4845e)

involving IHB (N–H58���O6). Besides, charges were

-1.0304e for nitrogen and -1.1887e for oxygen atom (O6).

A segment of NBO output summarizes the net atomic

charge distribution in terms of natural population analysis

(NPA). Summing all the populations for all orbitals on a

single atom and then subtracting the nuclear charge gives the

partial charge on each atom. The H58 was shown extra

depletion of electron density amounts to ?0.4661e and the

charges were -0.5564 for nitrogen and -0.71305 for oxygen

atom (O6) in intramolecular N–H58���O6 hydrogen bonding.

In Mulliken charge analysis, it was found that all

hydrogen accumulate positive charge as a result of molec-

ular relaxation and differ widely between 0.1148e and

0.3799e. The excess was taken from nearby hydrogen

(qH58 = ?0.3799e) involving in intramolecular N–H58���O6

hydrogen bonding. Negative charges around nitrogen (-

0.016710) and oxygen (-0.473105) with hydrogen link

were promoted the formation of deep electrostatic potential

wells in the neighborhood, thus pointing out chemical active

site for an IHB.

The CHELPG was made nearly the same partial charge

values as Mulliken [37, 59]. For the intramolecular N–

H58���O6 hydrogen bonding, the transfer of electronic charge

from acceptors to the donor amounts to 0.2522e. Partial

charges around nitrogen and oxygen were -0.5567 and -

0.1216, respectively.

All four methods were confirmed the H58 atom, linked to

the N atom, accumulate extra positive charge and refer to a

strong intramolecular donor–acceptor interaction in

Table 5 Partial charge (q in units of e) of the hydrogen atoms as

calculated at the B3LYP/6-311??G** level of theory

H-number qNPA qAIM

58 H 0.4661 0.4845

59 H 0.2380 0.0107

60 H 0.2303 0.0258

56 H 0.2362 -0.0009

51 H 0.2432 0.0223

50 H 0.2437 0.0189

49 H 0.2490 0.0332

47 H 0.2492 0.0352

45 H 0.2690 0.0722

40 H 0.2441 0.0291

38 H 0.2450 0.0281

39 H 0.2485 0.0315

36 H 0.2747 0.0763

34 H 0.2516 0.0401

27 H 0.2445 0.0191

28 H 0.2428 0.0240

29 H 0.2423 0.0163

2 H 0.2819 0.0234

12 H 0.2116 0.0319

13 H 0.2084 0.0146

14 H 0.2154 0.0317

17 H 0.2206 0.0380

18 H 0.2128 0.0323

23 H 0.2479 0.0366

25 H 0.2542 0.0404

Fig. 6 A comparison of NBO and AIM methods in charge contri-

bution of the hydrogen atoms in product 1-(Z). The H58 atom in

intramolecular N–H58���O6 hydrogen bonding accumulated extra

positive charge in four methods in comparison with other IHB

Struct Chem (2013) 24:623–635 629

123

N–H58���O6 as IHB in comparison with the other intramo-

lecular (see Fig. 6). An examination of the change in the

charge distribution for the hydrogen of the N–H link

obtained from the AIM and NPA methods as a function of

the CC CN dihedral angle change are given in Fig. 7.

(An examination of comparative results from CHELPG and

Mulliken analysis is given in the supporting information.)

The slight decreasing of the charge contents was observed

Fig. 7 Change in the calculated

charge distribution at the

B3LYP/6-311??G** level for

the hydrogen of the N–H link in

a intramolecular N–H58���O6

hydrogen bonding obtained

from the AIM and NPA

methods as a function of the

CC CN dihedral angle

change

Z -isomer

E -isomer

Z -isomer

TS1

TS2

(b)

(a)

Fig. 8 a The relative energy differences as a function of the

P–C C–O6 dihedral angle change from 0� to 180� with the

interval of 4� calculated at HF/6-311??G** level, the energies for

TS, Z-, and E-isomer computed at B3LYP/6-311??G** level of

theory including the zero-point scaled correction. b The energy profile

including TS, Z-, and E-isomer points, and structures corresponding to

TS, Z-, and E-isomer

630 Struct Chem (2013) 24:623–635

123

in the rotamer coming from weakening of the IHB and

lengthening of the H58���O6 bond. The curves obtained by

NBO and AIM methods were suited together and shown

good correlations between the charge distributions changes

versus dihedral angles. The same situation could not be

observed for the other IHB (weak IHB). This result was

indicated that existence of N–H58���O6 as a relatively strong

IHB was sensitive to any changes in product 1-(Z).

The barriers in bond rotation

For better understanding of the nature of the IHB, it should

be useful to explore the barriers of rotation around N–H���Obond. In order to determine theoretical rotational energy

barrier in the rotational interchangeable processes, the

optimized structures of ground-states (GS) and TS of

product 1-(Z) followed by the calculations of harmonic

frequencies which were carried out at B3LYP/6-

311??G** level of theory. The structures were obtained

by changing the P–C C–O6 double bond and CC CNsingle bond dihedral angles and applying the scan method

at HF/6-311??G** level of theory. The P–C C–O6and CC CN dihedral angles change were ranging from

0� to 180� and 0� to 360� with the interval of 4�, respec-

tively. Relative energies for each optimized point versus

dihedral angles and energy profiles are plotted in Figs. 8

and 9, respectively. As can be seen, only two TS are

appeared on the maximum points of the P–C C–O6rotation diagram in Fig. 8, whereas CC CN rotation

diagram in Fig. 9 shows a variation conversion path. In

these Figures, the barrier height on the Z-isomer is higher

than that on the E-isomer. That means the rotation barrier

energy of the Z-isomer involved in intramolecular hydro-

gen bond is higher than the reverse conversion process. The

maximum of Gibbs free rotational barriers in

GS1GS2

TS1

TS2

TS3

TS4

TS5

TS6

TS7

TS8

(a)

(b)

0

8.92

4.75

3.48

4.53

5.41

6.99

5.1

3.7

0.150

1

2

3

4

5

6

7

8

9

10

0 50 100 150 200 250 300 350 400

En

erg

y D

iffe

ren

ce (

kcal

/mo

l)

CC-CN Dihedral Angle (Degree)

HF B3LYP

Fig. 9 a The relative energy differences as a function of the

CC CN dihedral angle change from 0� to 360� and with the

interval of 4� calculated at HF/6-311??G** level, the energies for

GS and TS computed at B3LYP/6-311??G** level of theory

including the zero-point scaled correction. b The energy profile

including GS and TS points, and structures corresponding to GS1, GS2,

and TS1

Struct Chem (2013) 24:623–635 631

123

P–C C–O6 and CC CN dihedral angles changes

were equal to 31.12 and 9.27 kcal/mol, respectively. The

calculated free Gibbs rotational barriers (DG=), DH=, and

DS= including the zero-point energy (ZPE) scaled cor-

rections are presented in Table 6.

The N–H, H���O, and N���O bond lengths for the opti-

mized structure (GS1) at the B3LYP/6-311??G** level of

theory were 1.015 (0.995 at the HF/6-311??G** level),

2.001 (2.114 at the HF/6-311??G** level), and 2.824

(2.874 at the HF/6-311??G** level) A, respectively.

Also, the angle between the H���O and N–H link in the GS1

(N–H���O6 angle) was 135.5 (131.8 at the HF/6-311??G**

level) degree at the B3LYP/6-311??G** level, which are

favorable for an IHB. The calculated bond lengths profiles

at HF/6-311??G** level of theory by the scan method rest

dihedral angles are drawn in Figs. 10 and 11. As can be

seen, the distance between oxygen and hydrogen (H���O6)

and between nitrogen and oxygen (N���O6) shown in figure

enlarge along the dihedral changes with the consequent

weakening of the IHB, and vice versa. Diminution of the

Table 6 Rotation energy

barrier and activation energy

parameters for the

P–C C–O6 and

CC CN rotational

process in product

1-(Z) (gaseous phase)

corresponding to

Figs. 6 and 7, including

zero-point energy correction

The rotational process

CC CN DG= (kcal/mol) DH= (kcal/mol) DS=(cal/mol)

GS1 ? TS1 9.27 7.54 -5.80

TS1 ? TS2 -4.49 -3.77 2.40

TS2 ? TS3 -2.11 -0.70 4.70

TS3 ? TS4 1.11 0.30 -2.70

TS4 ? TS5 0.23 1.54 4.40

TS5 ? TS6 2.32 0.90 -4.80

TS6 ? TS7 -2.21 -1.03 3.90

TS7 ? TS8 -0.92 -2.20 -4.30

TS8 ? GS2 -2.15 -0.97 4.00

The rotational process

P–C C–O6 DG= (kcal/mol) DH=(kcal/mol) DS=(cal/mol)

Z-isomer ? TS1 31.12 28.61 -8.40

TS1 ? E-isomer -17.49 -16.10 4.70

E-isomer ? TS2 11.58 11.00 -2.00

TS2 ? Z-isomer -23.18 -22.50 2.30

Fig. 10 Changes in N–H bond

length as a function of the

CC CN and

P–C C–O6 dihedral

angles

632 Struct Chem (2013) 24:623–635

123

electron transfer from the lone pairs of oxygen to the

antibonding of hydrogen during dihedral changes was led

to the contraction of the N–H and elongation of the O���Hbond. This agrees with the suggestion made by Hobza and

Havlas [61] that the charge transfer from lone pairs of the

electron donor is directed mainly to the antibonding orbital

in the remote part of the molecule, which causes the

elongation in that part of molecule. Herein, Fig. 10 shows

how length diminishes on the N–H bond versus dihedral

angle changes.

Conclusion

The existence and symptom of the IHB in synthesized

2-substituted pyrrole stable phosphorus ylide have been

analyzed by the GIAO, AIM, NBO, and scanning methods.

The results were shown that Z-isomer as a single with

relatively strong intramolecular N–H���O6 hydrogen bond-

ing was the most stable according to the experimental

approaches.

In order to suggest a convenient and consistent protocol

to be employed for mimicking the experimental 1H NMR

spectra of synthesized ylide (product 1-(Z), Fig. 1); dif-

ferent single-point 1H chemical shift calculations were

considered using the same basis set. The one-parameter

hybrid mPW1PW91 functional using the 6-31G** basis set

was afforded the best overlap between calculated and

experimental outcomes. The detected extraordinary high-

frequency shift of the N–H bond signal with respect to the

relatively strong intramolecular N–H���O6 hydrogen bond-

ing in the 1H NMR spectrum was theoretically confirmed.

In a systematical study of N–H bond rotation, decrease of

the chemical shifts was consistent with an increase of the

lengthening of N���O6 and decrease in the IHB strength.

The analyses of the charge density and its Laplacian, a

negative total energy density at the BCP, estimated by AIM

calculations, were satisfied the indicative criteria of IHB

interactions, such N–H���O6, which according to classifi-

cation of Rozas et al. was medium-strength hydrogen bond.

This conclusion was clearly supported by the NBO results,

which lone pairs of oxygen atom participate as donor and

the r*(N–H) antibond as acceptor in conventional intramo-

lecular charge transfer interaction. As a consequence, the

occupation number of the r*(N–H) antibond was increased

considerably, while the occupation number of oxygen lone

pairs was decreased. This was confirmed the existence of a

strong hydrogen bonding interaction between the hydrogen

of the N–H link and O6 atom. The charge distribution

analysis obtained from the AIM, NPA, CHELPG, and

Mulliken methods for the hydrogen atoms were compared.

All four methods were confirmed the H atom, linked to the

N atom in a intramolecular N–H���O6 hydrogen bonding,

accumulate extra positive charge with respect to the other

hydrogen bonding interactions and refer to donor–acceptor

interaction as IHB. An examination of the change in the

charge distribution for the hydrogen of the N–H link, using

the AIM, NPA, CHELPG, and Mulliken analyses, as a

function of the dihedral angle rotation was revealed slight

decreasing of the charge contents coming from weakening

of the IHB and lengthening of the H���O bond. The recent

result along with the other data was indicated that a par-

ticular intramolecular N–H���O6 hydrogen bonding has a

remarkable role on the stereochemistry of 2-substituted

pyrrole which was appeared as a single isomer with

Z-orientation of carbon–carbon double bond. In the

Fig. 11 Changes in N–O and

H–O bond lengths as a function

of the CC CN and

P–C C–O6 dihedral

angles

Struct Chem (2013) 24:623–635 633

123

absence of the particular IHB, two geometrical isomers (2-

(Z) and 2-(E)) were possible that is shown in Fig. 1 for

previous work (N-substituted pyrrole).

The barriers in P–C C–O6 double bond and CC CNsingle bond rotation were theoretically explored in detail

and shown the existence of the conventional intramolecular

N–H���O6 hydrogen bonding was dictated appreciable

barriers of rotation. The IHB of the Z-isomer was raised the

free energy of activation, DG= = 9.269 kcal/mol in

CC CN dihedral angle changes and 24.192 kcal/mol in

P–C C–O6 dihedral angle changes, for rotation Z ? E.

Acknowledgments We gratefully acknowledge the financial sup-

port from the Research Council of the University of Sistan and

Baluchestan.

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