Scientific paper
Tautomerism of (3-Phenyl-allyl-) (5-pyridin-2-yl-[1,3,4]thiadiazol-2-yl) amine
Leokadia Strzemecka
Department of Organic Chemistry, Faculty of Pharmacy, Medical Academy of Lublin, Staszica 6, 20-081 Lublin, Poland * Corresponding author: E-mail: leokadia.strzemecka@am.lublin.pl
Received: 19-02-2007
Abstract
The radical and ionic structures of (3-phenyl-allyl-) (5-pyridin-2-yl-[1,3,4] thiadiazol-2-yl)-amine 2A(I) < 2A(I)' < 2A(I)'a, 2A(II) < 2A(II)' < 2A(II)'a have been determined by means of its (100 MHz, 500 MHz) 13C and 15N NMR spectra and B3LYP/6-31G** computations. The tautomeric equilibrium of 2A(I)' ^ 2B', 2A(II)' ^ 2C(II)' has been observed in the 1H NMR spectra (100 MHz)
Keywords: (3-phenyl-allyl-) (5-pyridin-2-yl-[1,3,4] thiadiazol-2-yl)-amine; electronic structure, tautomerism
1. Introduction
(5-Pyridin-2-yl-[1,3,4] thiadiazol-2-yl)-amine bearing allyl-(1) and (3-phenyl-allyl-) (2) substituents, type a tautomer exist as ionic and radical forms due to the changes of the electronic structure of the endocyclic nitrogen atoms of 1,3,4-thiadiazole and pyridine rings (Figs 1-3)1.
The XRD data confirm only one tautomer (a-type) in the crystals of both compounds 1 and 2. In the solid state the exo-amino form a is stabilized by different H-bonds, and the differences in the total energy between a
and b tautomers, are equal to -35.6 and -34.3 kJ/mol for 1 and 2, respectively according to the DFT level of theory calculations2. The 1H, 13C-and 15N NMR studies on the structure of allyl- (5-pyridin-2-yl-[1,3,4] thiadia-zol-2-yl)-amine 1a support the changes of the amine-type a nitrogen atom N-6 to pyridine-type A and pyrrole-type A(I). Previous 100 MHz 1H NMR investigations of 1 in the solution in the range from 5 8.665 to 7.233 of the chemical shift of N-H proton support the tautomeric equilibrium between allyl-(5-pyridin-2-yl-[1,3,4] thiadiazol-2-yl)-amine 1A 1A', 3H allyl- (5-pyridin-2 yl-[1,3,4] thiadiazol-2-ylidene)-amine 1B 1B'
U
12
m ,N
3
-N
H

13
15
14
H
V
N
\ H
H
8/
C-R
r
H
II
12
in
li
4
N-
H
-V
15 14
H
H -V
«/ =N
H
8/ -C
VR
H
R= -H (1)
R =
17 18
¡9 (2)
21 20
Fig 1: The tautomers a and b of allyl- (1) and (3-phenylal-lyl)-(2) (5-pyridin-2-yl-[1,3,4]thiadiazol-2-yl)-amine with atom numbering.
Fig 2: The tautomers a' and b', c' of allyl- (1) and (3-phenyl-allyl)- (2) (5-pyridin-2 -yl-[1,3,4]-thiadiazol-2-yl) -amine with atom numbering.
and 4H-allyl-(5-pyridin-2-yl-[ 1,3,4] thiadiazol-2-yli-dene) amine 1C'1.
The intensities of the signals of N-H proton point to the interconvertions of the 1A'5 ^ 1B3 ^ 1C'4 as well as to the balance of 1A'7 ^ 1B'7 and 1A'7 ^ 1C'7 tautomers and support pyridine-type nitrogen atoms N-10 N-4 N-6 and the amine-type nitrogen atoms N-4 N-3 of 1,3,4-thia-diazole ring1, respectively.
The aim of the present paper was to describe the electronic structure of the nitrogen atoms of 2a tautomer in the range from 5 13.64 to 7.233 of the chemical shifts of the N-H proton and its interconvertions to the imino forms in the solution in order to gain further insight into the structural features which determine biological activity. The 6-N and/or 5-substituted 2-amino[1,3,4]thiadiazole derivatives have exhibited activity against the leukemia, melanoma, lung carcinoma. They are also applied as the carbonic anhydrase inhibitors, and some of them show the antimycobacterial, anesthetic, antidepressant and anxi-olytic activity3-13. The 2-amino-[1,3,4] thiadiazoles are used as herbicides14, acting via inhibition of the imida-zoleglycerol phosphate dehydrase, as well as the corrosion inhibitors15. The screening biological test of 3-phenyl-allyl- (5-substituted-[1,3,4] thiadiazol-2-yl)-amine has been performed under the auspices of the Developmental Therapeutics Program, Division of Cancer Treatment, National Cancer Institute, Bethesda, Maryland, USA. They have been tested against the P 388 Leukemia Tumor Test System (3PS31): Leukemia Screening Test Result (LSTR) Raport and Screening Data Summary (SDS) Raport. The presumptive activity has been confirmed by SDS Raport but they have been inactive at dose levels tested, LSTR Raport. They have been tested for in-vitro anti-HIV activity, they have been inactive.
Fig 3: The resonance structures of allyl- (1) and (3-phenyl-allyl)-(2) (5-pyridin-2-yl)-[1,3,4]thiadiazol-2-yl)-amine
2. Experimental
2.1 General
The product 2 was prepared according to the published method16 and its NMR spectra (1H, 13C, 15N) were recorded under various conditions on Tesla BS 677 A and Bruker AM 500 spectrometers.
The 1H NMR spectra 7-10 of product 2 were measured with Tesla BS 677 A spectrometer (100 MHz with T.F.) in CDCl3 or DMSO solutions at room temperature with TMS as the internal standard. The 1H (spectrum 86), 13C and 15N NMR measurements of 2 were taken in CDCl3 and in DMSO-d6 solutions, respectively on a Bruker AM 500 spectrometer, operating at 500.18 MHz for hydrogen, 125.76 MHz for carbon and 50.68 MHz for nitrogen, using standard conditions. The 2D spectra of 1H 13C HMQC, 1H 13C HMBC, 1H 1H COSY have been recorded in CDCl3 solution at 500.18 MHz according to procedure given in Brucker programme library. Chemical shifts are given in 5 scale.
The 1H NMR spectra 81-4 have been recorded, ap-playing various concentration of product 2 in a DMSO or CDCl3 solution:
-	in a DMSO solution, the concentration of product 2 amounts to (1:3) spectra 81 82, respectively
-	in CDCl3 solution, the concentration of product 2 amounts to: 9 mg/0.5 ccm, spectrum 83, 18 mg/ 0.5 ccm, spectrum 84.
The 1H NMR spectra 7-10, 85, 86 have been recorded in a CDCl3, 87 in DMSO-D2O solutions without any determination of the concentration of 2 product.
The molecular geometries and properties corresponding to the local minima of the energy were calculated 2 at the DFT level of the theory with the B3LYP functional and the 6-31G** basis set.17,18 The same basis set and functional were used for the 1H, 13C and 15N NMR shielding constants calculations by applying the GIAO CPHF methods. The atomic charges were taken from the ESP fit using Breneman model (CHELPG). The Gaussian 98 package19 was employed for these calculations.
3. Results and Discussion
The calculated chemical shifts of the nitrogen atoms 15N for type a and type b tautomers of allyl- (1) (3-phenyl-allyl-) (2) (5-pyridin-2-yl- [1,3,4] thiadiazol-2-yl)-amine occur in different ranges: from about 5 - 309 to about -23 for type a tautomer and from about 5 - 225 to about -80 for b - one (Table 1, Fig. 4).2
The amino N-6 atom is strongly shielded in 1 (about 5 - 308) but in 2 the shielding decreases of a few ppm (to about 5 - 304). The shielding constants for the N-3 and N-10 atom in the 1,3,4-thiadiazole and pyridine rings, respectively are almost equal whereas N-4 atom is much less shielded.2
Table 1: Calculated 15N- and 'H-NMR chemical shifts 5 [ppm] of type a and b tautomers
Comp.	15n		1H	
1a 2a	-309	- -23		
1a	N6	-131.57	H14	8.125
	N3	-77.78		
2a	N10	-86.0	H6	7.5
	N10	-72.36	H6	6.45
	N6	-133.98		
1b 2b	-225	- -80		
Fig 4: The linear regression of shielding constants c [ppm] versus chemical shifts 5 [ppm] for 1a and 2a
In the 1H NMR spectra of 2 the nitrogen atom N-6 appear as amine-type a, pyridine-type A, pyrrole-type A(I) and in sp hybridization A(II) tautomers (Figs 1-3). The calculated chemical shift value for the proton of N-H group at 5 6.45 (Table 1)2 points to the amino proton of 2a tautomer slightly shifted by the weak intermolecular interactions of the solut-solvent type. The signal of the nitrogen atom 15 N appears at 5 - 304.07. The calculated chemical shift of N-6 at 5 - 133.98 (Table 1)2 supports pyridine-type nitrogen, 2A tautomer. The calculated chemical shift of N-H proton at 5 7.5 (Table 1)2 supports sp2 or sp hybridization of N-6, 2A, 2A(I), 2A(II) tautomers and the lack of the charges over 1,3,4-thiadiazole ring.
The coupling constants J(H7H8) 6.2 Hz (500 MHz),2 J(H8H9B) 15.8 Hz, J(H9BH8) 15.87 Hz, J(H8H9A) 12.6 Hz, J(H9AH8) 12.6 Hz (100 MHz)20 confirm pyrrole-type nitrogen atom N-6, 2A(I) tautomer whereas J(H7DH7C) 1.4 Hz (500 MHz),2 J(H8H9B) 15.9 Hz, J(H9BH8) 15.9 Hz (500 MHz, 100 MHz),220 J(H8H9A) 13.1 Hz, J(H9AH8) 13.1 Hz (100 MHz)20 the sp hybridization of N-6, 2A(II) tautomer. The coupling constants J(H7CH8) 5.9 Hz, J(H7DH8) 5.7 Hz and J(H7CH8) 9.5 Hz, J(H7DH8) 9.2 Hz (100 MHz)20
support the transformation of sp2 < sp hybridization of N-6 of the rigid structures.
The 13C NMR resonances of 3-phenyl-allyl radical C-9 at 5 133.52, C-8 at 5 123.83, C-7 at 5 49.07 and of the benzene C atoms C-16 at 5 136.20, C-19 at 5 127.97, C-18, C-20 at 5 128.60 as well as the chemical shift of the proton of phenyl group of 3-phenyl-allyl substituent H-19 at 5 7.192 (mult.)2 confirm the positively charged cin-namyl cation. The signals of H-17, H-21 at 5 7.314 (d) and C-17, C-21 at 5 126.56 support the conjugated bonds of cinnamyl substituent. The resonances of H-18, H-20 arise at 5 7.239 (td).
The calculated signal of H-14 at 5 8.125 (Table 1)2 as well as the 1H 1H coupling constants J(H12H14) 1.0 Hz, J(H11H14) 0.5 Hz of 1a tautomer2 confirm the absence of the charges on the pyridine ring. The calculated chemical shift of N-3 at 5 - 77.78 (Table 1)2 confirm pyridine-type nitrogen atom of 1a tautomer and the lack of the differences in the spin states of electrons of 2p orbitals of N-3 C-2. The calculated chemical shift of N-10 at 5 - 86.0 of 2a tautomer (Table 1)2 point to the amine-type nitrogen atom.
The 1H 13C HMQC correlation spectra of 2 show a correlation signal between H-14 at 5 8.290 and C15 at 5 149.7. The above data prove the diradical resonance structures a0c A(I)0c A(II)0c, a0e A(I)0e A(II)0e (Fig 3) and the lack of the charges over pyridine and 1,3,4-thiadiazole rings. Pyridyl H-14 proton of the diradical resonance structures a0c A(I)0c A(II)0c, a0e A(I)0e A(II)0e is more in-tensly deshielded about 0.15 ppm in relation to the structure a A(I) A(II). The spectroscopic data support the conjugation of aromatic n electrons of pyridyl substituent
with n electrons of double C = N bond of 1, 3, 4 thiadia-zole ring in solution.
a; AsA(1);A(I[)5
Fig. 5: The resonance structures of the pyridyl substituent
Table 2: !H NMR chemical shifts 5 [ppm] from TMS of 2.
Spectrum No
H 7
H 8 H 9
Benzene H atoms
Pyridin - 2- yl
8j (DMSO)
4.218 - 4.115 2H m
6.771 - 6.248 2H m
7.522 - 7.224 5H m
8.635 - 8 8.142 - 8 8.003 - 7 7.522 - 7
.560 1H	H11
.037 1H	H13 H14
.835 1H	H12 H13
.224 1H	H14 H12
82 (DMSO)
4.242 - 4.147 2H m
6.788 - 6.265 2H m
7.530 - 7.232 5H m
8.650 - 8 8.169 - 8 8.010 - 7 7.530 - 7
.574 1H	H11
.067 1H	H13 H14
.842 1H	H12 H13
.232 1H	H14 H12
83 (CDCl3)
4.232 - 4.161 2H m
6.805 - 6.168 2H m
7.527 - 7.193 5H m
8.591 - 8 8.213 - 8 7.830 - 7 7.527 - 7
.513 1H	H11
.110 1H	H13 H14
.659 1H	H12 H13
.193 1H	H14 H12
84 (CDCl3)
4.215 - 4.147 2H m
6.785 - 6.165 2H m
7.447 - 7.129 5H m
8.574 - 8 8.179 - 8 7.798 - 7 7.447 - 7
.499 1H	H11
.076 1H	H13 H14
.627 1H	H12 H13
.129 1H	H14 H12
87 (DMSO + D2O) 4.220 - 4.169 2H m
6.785 - 6.251	7.527 - 7.207	8.650 - 8.577 1H H11
2,4H m	5H m	8.164 - 8.089 1H H13 H14
8.032 - 7.864 1H H12 H13 7.527 - 7.207 1.4H H14 H12
The signals of the N-H proton and the pyridyl substituent in the *H NMR spectra (100 MHz) support the a A(I) A(II), a15 A(I)15 A(II)15 and radical resonance structures a' A(I)' A(II)' a'1-8 aA(I)'1-8 A(II)'1-8 a'0 A(I)'0 A(II)'0 (Figs 1-3, 5, Tables 22-10).
In the 1H NMR spectra of 2 (100 MHz) in the range from 5 8.650 to 5 7.233 of the chemical shifts of N-H proton, the nitrogen atoms N-3 N-4 N-10 appear as pyridine-type, pyrrole-type and amine-type nitrogen while N-6 as pyrrole-type, structures A(I) A(I)'A(I)0 or in sp hybridization, structures A(II) A(II)'A(II)0 (Fig 3).
The absence of the charges over 1,3,4 thiadiazole ring confirm the lack of the transition of electrons of p orbitals of 1S 2C 3N 4N 5C of 1,3,4-thiadiazole ring. The changes of the electronic structure of the nitrogen atoms N-3 N-4 N-10 (Fig. 3) have been described previously1. The 1H 1H long-range coupling constants in the 37.376 Hz - 43.520 Hz range (spectra 7-10)20 (Table 8), support the coupling of the protons of the pyridyl and -N-CH2-CH=CH-C6H5 groups via 2p orbitals of C-14 C-7 of the rigid structures A(II)'A(II)'a and sp hybridization of the exocyclic nitrogen atom N-6 (Fig. 6).
In the 2D 1H 13C HMQC correlation spectra the signals of H-11 at 5 8.490 and H-14 at 5 8.0802 exhibit a correlation to C-12, C-8 at 5 123.8. The 2D 1H 13C HMQC correlation spectra show the cross-peaks of H-9A at 5 6.600, H-9B at 5 6.650 as well as the correlation signals of H-8A at 5 6.220, H-8B at 5 6.250 to C-16, C-13 at 5 136.2. Such long-range couplings can be observed if the bonds assume the planar configuration. In the 2D 1H 13C HMQC correlation spectra the correlation signals of H-7
at 5 4.15 to C-8, C-12 at 5 123.8, C-16, C-13 at 5 136.2, C-2 at 5 171.5 support the planar structure.
In the 2D 1H 13C HMBC correlation spectra the cross-peak of H-7 at 5 4.100 to C7 at 5 49.00 is observed. In the 2D 1H 13C HMQC correlation spectra the signals of H-6 at 5 4.200 and 5 4.000 exhibit a correlation to C-7 at 5 49.1 and support a tautomer. The signals at 5 0.498-4.266 (Table 8, spectra 10, 7) support the transformation of sp O sp.3
The differences in the resonances of N-H proton in the range from 5 8.650 to 7.233 are caused by the atomic charge over the pyridine ring.
To assigne the resonance structures of 2 in the range from 5 8.650 to 5 7.233 of the chemical shifts of N-H proton, the 13C, 15N and 1H resonances line in 13C, 15N and 1H NMR spectra (100 MHz, 500 MHz) of 2 and the coupling constants of the pyridyl substituent have been analyzed.
Fig 6: The resonance rigid structures A(II)', A(II)'a of (3-phenyl-allyl)- (5-pyridin-2-yl-[1,3,4] thiadiazol-2-yl)-amine
Table 3: The !H NMR chemical shifts 8 [ppm] from TMS of 2.
Spectrum No
H 7
H 8 H 9
Benzene H atoms
Pyridin - 2- yl
7(CDCl3)
4.266 - 4.210 2H
6.430 - 6.153 1H
6.815 - 6.660 1H
7.444 - 7.242 5H
8.580 - 8.533 1H	H11
8.176 - 8.096 1H	H13 H14
7.890 - 7.674 1H	H12 H13
7.444 - 7.242 1H	H14 H12
8(CDCl3)
4.224 - 4.163 2H
6.416 - 6.144 1H
6.782 - 6.622 1H
7.430 - 7.190 5H
547 - 8 .143 - 8. 796 - 7 430 - 7
500 1H 063 1H 627 1H 190 1H
H11
H13 H14 H12 H13 H14 H12
85 (CDCl3)
4.2 2H
6.72 - 6.12 2H
7.280 5H
48 1H 08 1H .64 1H 28 1H
H11
H13 H14 H12 H13 H14 H12
9 (CDCl3)
4.252 - 4.182 2H
6.801 - 6.641 1H
6.421 - 6.144 1H
7.448 - 7.209 5H
570 - 8 .162 - 8. .829 - 7. 448 - 7
519 1H 082 1H 655 1H 209 1H
H11
H13 H14 H12 H13 H14 H12
10 (CDCl3)
4.257 - 4.196 2H
6.646 - 6.134 2H
7.448 - 7.233 5H
570 - 8 .162 - 8. 838 - 7 448 - 7
523 1H 082 1H 669 1H 233 1H
H11
H13 H14 H12 H13 H14 H12
«j.	2
.¡a	K
?	*
k	Z
-	S
o Z
S s
13
v a ce
m, ^ ^
î £ S
« NN
I ^ « BO I x
ti G „ —
^^ fi TT /—-
<	O O ti « « ^ ^ ^
ÎT ^ Tf »s
^ A A A
— ^ ^ ^ ^r^ ^
^ ^ ^ * ^ NN ^
NN	NN ^ NN
W^NN	NN NN w W
<	rt^ rs^rs^	^ ^ rs*^ rs'
TT rt	NN
*Í5 *Í5 áP ^ g
TT	TT			TT		rt	rt
NN TT	NN TT	hH	hH	NN TT	hH	NN NN ^ rt	NN
NN	NN	NN	NN	NN	NN	NN a	NN a
	ci'	a	2a	ci'	a		
o	••o	o	(N "	••o		<ri	
	c^						<ri
		o					
00	00	00	00	00	00	00	00
1 <n	1	1	1 (N	1 o\	1 o\	1 <ri	1 (N
							
CM	1—1	1—1	1—1	1—1	1—1	i—i	i—i
00 00 00 00 00 00 00 00
o o hH hH	o NN				O NN
,,					,
N^ N^	NN				N^
O o	O				O
					
III oo rt		TT	00	rt	I rt tt
NN NN	NN	NN	NN	NN	NN NN
oo rt		TT	00	, 5>	rt tt
NN NN ^ ^ oo rt	NN	NN < TT	NN 00	NN	NN NN ^ ^ TT
aaaaaaaa
tttttt^TTTTI^IT
tttttt^TTTTI^IT
tttttt^TTTTI^IT
Q O
+ U _ . O Q 30
U^j Q 2
co"pí co" Co" co" 00 00
m
s
h Ë
e &
-s
S
!Z à
-C
H

•!3 «
- s
o Z
5
| e
Iii:
6	o ce ce
5 S ^ < <
»'0 ^hH ö ö
g Ô»^ ^
^ W	TT O
* «
« « ^ tt II « <
00 00 T NN NN NN rs — NN ^^S'NNNNNN^NN
H B
<i « « « ^ « « « «"III«!
6 6 N 1" _<r
III
rr rr NN NN
TT TT
CS-CS-CS-ÖÖÖCS-Ö ^ < < ^
TT TT »s »s »s ^ & && && &&
	(N	in		o		in	
		en			in	in	IN
				VO		VO	vi^
	^	^					
(N	O	en	O	"	O		"
<ri	1—1	o	o\	<n	<n	IN	o\
O	o		00				
00	00	00					
<
a
TT O '
^ ^ ^
< ^ l TT O *
		± ±			
		£	TT	y y	
		NN NN	NN NN		^^ NN
hH ,	NN ,	^	^	NN ,	NN ^^
		hH	NN		
N^	N^		TT	N^	
		î Î			^ «
I t		rt	rs'	III	
	TT NN ^ NN		N^	rt	
NN	,		,	NN	Èîti
	TT	N^	N^		< ^ rt rt
NN a	NN 2a			NN rt a	NN NN rt rt « «
oo- r _
CO^1 oo" C-^^H	CO"
iU
Q
C U

■5 &
S
:z sc
-C
H

o Z E jb
Iii:
& C
ce ce
<
a
			SD	I		O		
	NN NN		NN	y		NN	NN	
NN	^			NN	N^			
	NN		SD N^			O N^	N^	
NN <			< SD	NN	NN	O		
								
I	I		I			11		
		rs —		I I		00	TT	TT
NN	NN NN	NN	NN	rt	rt NN ^ NN		NN	NN
,	^	,	,	NN	,			
						00	TT	TT
				NN	NN	00	TT	< TT
aaaaaaaaa
						rt	rt	TT
NN	NN	NN	NN	NN	NN	NN	NN	NN TT
NN	NN	NN	NN	NN	NN	NN 5a	NN «	NN
a	«a	2a	2a	2a	5a			es"
		IT)	IT)	a\	IN	a\	O
o	IN	o\	IT)	o		IN	o\
CM	CM		CM	CM	CM	T-H	
1	P^ 1	P^ 1	P^ 1	p^ 1	P^ 1	P^ 1	p^ 1
p-	IN	P~					o
IN	IN	IN					IT)
			■it			■ít	
P^	P^	P^	P^	p^	P^	P^	P^
< NN
*Č3 t
<
a
^^NN^^NN
O 00 f
■^■rs rs rs
a aa
rt
a a a a
tttt
tr~ -H	tj
NNNNNNNN ^^^NNNNNNNN^
'JSCs
-H M	^^ ^—t ^—t ^—t ^—t
$ $

'
22ttttt
XX	'í
^ ^	SD SD SD SD SC
TT TT TT TT
^ ^ ^ ^
TTTTTTTT»s rs rs rs rs
aaaaaaaaa
		
	Q	
C5	C) C3	Ol
<jy	uy m	Q
S	S S	
Q	S s	
		
"	co" o"	oo"
^ G ^ Q
QUU
Table 7. The 1H-NMR chemical shifts 8 [ppm] from TMS of 2.
Spectrum No Solvent	H 11	
87(DMSO-D2O)	8.650-i	.577
82(DMSO)	8.650—	3.574
8j(DMSO)	8.635 -	8.560
83(CDCl3)	8.591-	.513
7(CDCl3)	8.580 -	8.533
84(CDCl3)	8.574-	.499
10(CDCl3)	8.570-	3.523
9(CDCl3)	8.570-	.519
8(CDCl3)	8.547-	.500
Pyridin-2- yl
structures
a5A(I)5A(II)5 ^ a'8A(I)'8A(II)'8 ^ a'4A(I)'4A(II)'4 ^ a'A(I)'A(II)' a5A(I)5A(II)5 ^ a'8A(I)'8A(II)'8 ^ a'4A(I)'4A(II)'4 ^ a'oA(I)'o A(II)'o a4A(I)4A(II)4 ^ a'4A(I)'4 A(II)'4 ^ a'sA(I)'s A(II)'S a2A(I)2A(II)2 ^ a'4A(I)'4 A(II)'4 ^ a'-A(I)'- A(II)'-a'4A(I)'4A(II)'4 ^ a'2A(I)'2A(II)'2 ^ a'8A(I)'8A(II)'8 ^ aA (I) A(II) a'3A(I)'j A(II)'j ^ a'1A(I)'1 A(II)'X a2A(I)2A(II)2 ^ a'2A(I)'2 A(II)'2 ^ a'A(I)'A(II)' a2A(I)2 A(II)2 ^ a'-A(I)'- A(II)'- ^ a'o A(I)'o A(II)'o a'sA(I)'s A(II)'S ^ a'2A(I)'2 A(II)'2
Table 8: The 'H'H long - range coupling constants [Hz] of 2 Spectrum
Table 10: The 'H NMR chemical shifts 8 [ppm] from TMS of NH proton of 2A(I)', 2A(II)' tautomers
No (CDCl3)	8	J		NH
10	0.498	J(HHn)	38.400 Hz	0.279 H
7	4.210	J(H7CH14	43.008 Hz	0.7 H
7	4.257	J(H7CH12	43.264 Hz	
7	4.266	J(H7DH13	41.472 Hz	
9	6.641	J(H9AH13	37.376 Hz	0.272 H
9	8.082	J(H13H9A	37.632 Hz	0.628 H
8	7.190	J(H14H9A	38.784 Hz	3.08 H
8	7.369	J(H14H9B	42.624 Hz	
10	8.523	J(H11H9A	38.912 Hz	0,107 H
8	8.063	J(H13H9B	42.496 Hz	0.742 H
8	7.702	J(H12H7C	43.392 Hz	3.425 H
7	7.242	J(H14H9B	43.520 Hz	2 H
9	7.228	J(H14H9B	43.520 Hz	1.965 H
Table 9: The 'H NMR chemical shifts 8 [ppm] from TMS of NH proton of 2A(I), 2A(II) tautomers
Spectrum No Solvent	8		NH	Structure
85 (CDCl3)	13.64		(s)	2A(I) 2A(I)'
82 (DMSO)	8.650 -	8.574	0.08 H	2A(II) 2A (II)'
8j (DMSO)	8.635 -	8.560	0.4 H	
10 (CDCl3)	8.570 -	8.523	0.107 H	2A(I)1 2A(II)1
9 (CDCl3)	8.570 -	8.519	0.236 H	2A(I)2 2A(II)2
8 (CDCl3)	8.547 -	8.500	0.61 H	
85 (CDC3l3)	8.48		0.25 H	
851 (DMSO3 )	8.435 -	8.345	1.08 H	2A(I)- 2A(II)-
821 (DMSO)	8.411 -	8,306	1.5 H (t)	2A(I)4 2A(II)4
The resonance structures of the pyridine ring are shown on Fig. 5.
In the 13C NMR spectrum of 2 the chemical shifts of C-11 at 8 149.33 and C-15 at 8 149.76 2 confirm pyridine-type nitrogen atom N-10, the structures a1 A(I)1 A(II)r a'1A(I)'1 A(II)'1, a'2 A(I)'2 A(II)'2 and a5 A(I)5 A(II)5 respectively.
The chemical shift of C-12 at 8 124.122 of 2 support the pyridine-type nitrogen atom N-10 of the structures a2 A(I)2 A(II)2, a'3 A(I)'3 A(II)'3, a'sA(I)'s A(II)'S. The sig-
Spectrum No Solvent	8		NH	Structure	
7 (CDCl3)	8.176	- 8.096	0.04 H		
9 (CDCl3)	8.162	- 8.082	0.628 H	2A(I)S 2a(ii)s	
10 (CDCl3)	8.162	- 8.082	0.358 H	2A(I)'1	2A(II)'1
8 (CDCl3)	8.143	- 8.063	0.742 H	2A(I)'2	2A(II)'2
85 (CDC3l3)	8.08		0.5 H	2A(I)'-	2A(II)'-
81 (DMSO)	8.003	- 7.835	0.6 H		
71(CDCl3)	7.890	- 7.674	2H		
10 (CDCl3)	7.838	- 7.669	2H		
83 (CDCl33)	7.830	- 7.659	0.15 H		
93(CDCl33)	7.829	- 7.655	2H		
8 (CDCl3)	7.796	- 7.627	3.425 H		
86 (CDCl3)	7.683	- 7.680	0.089 H		
85 (CDCl3)	7.64		2.5 H		
852 (DMSO3 )	7.530	- 7.232	0.4	2A(I)'4	2A(II)'4
8j (DMSO)	7.522	- 7.224	2.5 H	2A(I)'S	2a(ii)'s
831 (CDCl3)	7.527	- 7.193	0.7 H	2a(i)'s	2a(ii)'s
10 (CDCl3)	7.448	- 7.233	1.721H		
9 (CDCl3)	7.448	- 7.209	1.965 H	2A(I)'S	2a(ii)'s
7 (CDCl3)	7.444	- 7.242	2H	2a(i)'s	2a(ii)'s
8 (CDCl3)	7.430	- 7.190	3.08 H	2A(I)'7	2A(II)'7
84 (CDC3l3)	7.447	- 7.129	0.5 H		
86 (CDCl3)	7.323	- 7.306	0.165 H		
85 (CDCl3)	7.280		2 H		
856 (CDCl33)	7.252	- 7.174	0.457 H		
nal of C-14 at 8 119.892 point to the structures a3 A(I)3 A(II)3, a'4 A(I)'4 A(II)'4, a5 A(I)5 A(II)5. The signal of C-
13	at 8 136.802 confirms the structures a2 A(I)2 A(II)2, a-A(I)- A(II)-, a'- A(I)'- A(II)'-, a4 ACO4 AOD^ a'4 A®^ A(II)'4, a'5 A(I)'5 A(II)'5.The chemical shift of N-10 in 15N NMR spectrum of 1 at 8 - 74.78 supports the structures a2 A2 A(I)2 A(II)2, a'3 A'3 A(I)'3 A(II)'3, a4 A4 A(I)4 A(II)4, a'5-8 A'5-8 A(I)'5-8 A(II)'5-8.The calculated chemical shift of N-10 of 2 at 8 - 72.36"(Table 1)2 confirms the structures ax A(I)j A(II)j, a\ A(I)'j A(II)'j, a'2 A(I)'2 A(II)'2, a- A(I)- A(II)-, a'4 A(I)'4 A(II)'4, a5 A(I)S A(II)S. The 1H NMR spectrum 86 (500 MHz) shows a signal of H-
14	at 8 8.087 of the structures a3 A(I)3 A(II)3, a'8 A(I)'8 A(II)'8.
In the 13C HMBC and HMQC correlation spectra the signal of H-14 at 5 8.0802 exhibits a correlation to C-14 at 5 119.9 and C-12, C-8 at 5 123.8, C-15 at 5 149.7, C-5 at 5 160.0, respectively and confirms a'5 A(I)'5 A(II)'5. In the 2D 1H 13C HMQC correlation spectra the cross-peak between H-14 at 5 7.500 and C-13 at 5 136.8 support the resonance structures a3 A(I)3 A(II)3 o a'5 A(I)'5 A(II)'5. The cross-peak between H-14 at 5 7.200 and C-13, C-16 at 5 136.2 support the resonance structures a'8 A(I)'8A(II)V
In the 1H 13C HMBC and HMQC correlation spectra the signal of H-13 at 5 7.690 exhibits a correlation to C-13 at 5 136.8 and C-14 at 5 119.9, C-15 at 5 149.7, C-5 at 5 160.0, respectively and confirms a'3 A(I)'3 A(II)'3 o a2 A(I)2 A(II)2 and a3 A(I)3 A(II)3 O a4 A(I)4 A(II)40 a'4 A(I)'4 A(II)'4 resonance structures.
In the 2D 1H 13C HMQC correlation spectra the cross-peaks between H-13 at 5 8.220, 8.200 and C-14 at 5 119.9 support the resonance structures a\ A(I)\ A(II)'j O a'8 A(I)'8 A(II)'8. The cross-peaks between H-13 at 5 7.920, 7.900 and C814 at 5 119.9 support the resonance structures a4 A(I)4 A(II)4 o a- A(I)- A(II)-. The correlation signal between H-13 at 5 7.830 and C-15 at 5 149.7 confirms a4 A(I)4 A(II)4 o a3 A(I)3 A(II)3o a2 A(I)2 A(II)2 resonance structures.
In the 2D 1H 13C HMBC, HMQC correlation spectra a correlation signal between H-12 at 5 7.200 and C-12 at 5 124.1 as well as the correlation signals of H-12 at 5 7.200 to C-5 at 5 160, C-15 at 5 149.7, C-14 at 5 119.9 point to the resonance structures a'5 A(I)'5 A(II)'5.
The 2D 1H 13C HMB-C, HM- QC co-rrelation spectra
show a correlation signal between H-11 at 5 8.490 and C-11 at 5 149.3 as well as the correlation signals of H-11 at 5 8.490 to C-15 at 5 149.7, C-14 at 5 119.9, C-13 at 5 136.8, C-12, C-8 at 5 123.8 and point to the resonance structures a3 A(I)3 A(II)3 o a'3 A(I)'3 A(II)'3, a2 A(I)2 A(II)2 O a'4 A(I)'4 A(II)'4.
The 1H 1H coupling constants J(H13H12) 7.8 Hz, J(H12H13) 7.8 Hz 2 of 2a tautomer support the positive charge at C-13 atom of the structures a2 A(I)2 A(II)2. The coupling constants J(H14H13) 7.0 Hz, J(H12H11) 4.9 Hz, J(H11H12) 4.9 Hz, J(HnH14) 1.0 Hz, J(H14Hn) 1.0 Hz 2 point to the positive charge at C-15 atom and the negative one on N-10 atom of pyridine substituent of the structures a'8, A(I)'8 A(II)'8. The coupling constants J(H12H13) 1.7 Hz, J(H13H12) 1.7 Hz 2 of 2a confirm the lack of the charges on the pyridine ring.
In the 1H NMR spectra 7-10 the signals of N-H proton in the range from 5 13.64 to 8.480 of the chemical shifts support the structures 2A(I) 2A(I)', 2A(II) 2A(II)', 2A(I)1, 2A(II)1 2A(I)2 2A(II)2 (Table 9) (Figs 1-3, 5).
The signals of N-H proton in the range from 5 8.435 to 8.306 (Table 9) and at 5 8.176-8.063 (Table 10) confirm the resonance structures 2A(I)3, 2A(II)3 2A(I)4, 2A(II)4 and 2A(I)-, 2A(II)- 2A(I)'1 2A(II)'1 respectively. At 5 8.003-7.835 and at 5 7.830-7.627 2A(I)'1 2A(II)'1
2A(I)'2 2A(II)'2 and 2A(I)'3 2A(II)'3resonance structures arise, respectively. In the range of 5 7.530-7.193 and 5 7.448-7.129 the resonance structures 2A(I)'4 2A(II)'4 2A(I)'S 2A(II)'S 2A(I)'6 2A(II)'6 and 2A(I)'S 2A(II)'4 2A(I)'6 2A(II)'6 2A(I)'7 2A(II)'7 appear (Table 10).
In the 1H NMR spectra 81 82 (100MHz, DMSO) the signals at 5 8.390 (1.08H, degenerated broaded triplet) and at 5 8.358 (1.5 H, broaded triplet, Table 9) correspond to the N-H proton of 2A(I)3 2A(II)3 and 2A(I)4 2A(II)4 tautomers, respectively.
The broaded triplets suggest that these protons take part in the intermolecular hydrogen bonds. The broaded triplet in the 1H NMR spectrum 82 indicates the slow exchange of the N-H proton, due to this fact, the coupling of H-6 H-7 protons may be observed and support 2A4 2A(I)4 2A(II)4 tautomers. These signals are the averaged ones in consequence of the rapid transitions of hydrogen atom between the exocyclic nitrogen atom N-6 and N-3 N-4 ones of 1,3,4-thiadiazole ring, then degenerated broaded triplet at 5 8.390 in the 1H NMR spectrum 81 point to the 2A3 2B3 2C3 tautomers. They disappear in D2O (spectrum 87). In the 1H NMR spectrum 85 of product 2 recorded in CDCl3 solution at 100 MHz the considerable deshielding of the N-H proton at 5 13.64 indicates the possible intramolecular hydrogen bond and supports 2C' 2C(I)' 2C(II)' tautomers. In the 1H NMR spectrum 11 (100 MHz, DMSO) the magnitude of the couplings J(H8H7D) = J(H8H7C) 8.2 Hz support the changes of sp2 < sp3 hybridization of the nitrogen and carbon atoms N-6 C-7. The coupling constants of the protons J(H8H9B) 15.4 Hz, J(H8H9A) 8.5Hz, J(H8H7C) 7.6 Hz, J(H8H7D) 7.6 Hz support the sp3 hybridization of C-7 carbon atom.21
Fig. 7: The 'H NMR NH group signals at 5 7.890-7.674 and 5 7.444-7.242 (spectrum 7)
Fig. 8: The !H NMR NH group signals at 8 7.838-7.669 and 8 7.448-7.233 (spectrum 10)
The 1H NMR spectra of 2 (100 MHz) confirm the co-existence of two tautomeric forms A(I)' ^ B' or A(II)' ^ C(II)' in the solution.
The signals of N-H proton in the range from 8 7.890 to 7.674 (2H) and at 8 7.444 to 7.242 (2H) (Fig. 7, spectrum 7, Table 10) point to the transformation process of 2A(I)'1 ^ 2B'j, 2A(I)'6 ^ 2B'6 or 2A(II)'i ^ 2C(II)'i, 2A(II)'6 ^ 2C(II)'6, respectively.
The signals of N-H group at 8 7.838-7.669 (2 H) and at 8 7.448-7.233 (1.721 H) (Fig. 8, spectrum 10, Table 10) confirm the balance of 2A(I)'2 ^ 2B'2, 2A(I)'5 ^ 2B'5 or 2A(II)'2 ^ 2C(II)'2, 2A(II)'5 ^ 2C(II)'5, respectively.
The signals of N-H proton at 8 7.796-7.627 (3.425 H) and at 8 7.430-7.190 (3.08 H) (Fig. 9, spectrum 8, Table 10) point to 2A(I)'3 2A(II)'3 and 2A(I)'7 2A(II)'7 resonance structures and to the interconvertion of 2A(I)'3 ^ 2B'3, 2A(I)'7 ^ 2B'7 or 2A(II)'3 ^ 2C(II)'3, 2A(II)'7 ^ 2C(II)'7, respectively.
The signals of N-H proton at 8 7.64 (2.5 H) and at 8 7.28 (2 H) (spectrum 85, Table 10) confirm the 2A(I)'3 2A(II)'3 and 2A(I)'6 2A(II)'6 structures and the transformation process of 2A(I)'3 ^ 2B'3, 2A(I)'6 ^ 2B'6 or 2A(II)'3 ^ 2C(II)'3, 2A(II)'6 ^ 2C(II)'6,respectively.
Fig. 9: The 'H NMR NH group signals at 8 7.796-7.627 and 8 7.430-7.190 (spectrum 8)
Fig. 10: The 1H NMR NH group signals at 8 7.829-7.655 and 8 7.448-7.209 (spectrum 9)
The signal of N-H group at 5 7.829-7.655 (2 H) and at 5 7.448-7.209 (1.965 H) (Fig. 10, spectrum 9, Table 10) supports the balance of 2A(I)'3 ^ 2B'3, 2A(I)'5 ^ 2B'5 or 2A(II)'3 ^ 2C(II)'3, 2A(II)'s ^ 2C(II)'5 respectively. The signal of N-H group at 5 7.522-7.224 (2,5 H) (spectrum 81) indicates 2A(I)'4 2A(II)'4 structures as well as the transformation process of 2A(I)'4 ^ 2B'4 or 2A(II)'4 ^ 2C(II)'4. These transformation confirm the amine-type nitrogen N-4, N-3 of 1,3,4-thiadiazole ring.
The interconvertions of the structures 2A(I) < 2A(I)' < 2A(I)'a, 2A(II) < 2A(II)' < 2A(II)'a and the rapid exchange of the NH hydrogen suggest the proton transfer of 2A(I)' ^ 2B', or 2A(II)' ^ 2C(II)' tautomers via solvent. Double signals of the protons corresponding to both tautomeric forms are present in the :H-NMR (100 MHz) spectra of 2 (Figs 7-10, Table 10). The proton transfer reactions for different systems have been described in the literature.22,23
Table 11: The 1H-NMR chemical shifts 8 [ppm] from TMS of the NH group of tautomer 1A 1A'.
Spectrum No Solvent	5		NH	Structure
1j (DMSO)	8.637	- 8.562	0.08 H	11A 1A'
13 (CDCl3)	8.606	- 8.530	0.2 H	1Aj 1A2
14 (CDClj)	8.601	- 8.525	0.05 H	
3 (CDCl3)	8.598	- 8.537	0.23 H	
6 (CDCl3)	8.598	- 8.523	0.1 H	
1 (CDCl3)	8.594	- 8.519	0.38 H	
5 (CDCl3)	8.589	- 8.514	0.637 H	
2 (CDCl3)	8.580	- 8.537	0.08 H	
5(CDCl33)	8.077	- 7.974	0.756 H	1A'i
4(CDCl33)	7.852	- 7.683	0.13 H	1A'2
6(CDCl33)	7.852	- 7.678	0.14 H	1A'3
1(CDCl3)	7.847	- 7.674	0.43 H	
2(CDCl33)	7.847	- 7.674	0.18 H	
3(CDCl33)	7.847	- 7.674	0.25 H	
5(CDCl33)	7.838	- 7.646	1.356 H	
17(CDCl3)	7.78 -	■ 7.73	0.505 H	
Table 12: The 1H-NMR chemical shifts 8 [ppm] from TMS and the 1H-1H long-range coupling constants [Hz] of 1
Spectrum No (CDCLj)	5	J	NH
4	8.528	/(HuHA) 37.280	
6	8.598	/(HuHa) 38.144	0.1 H
1	7.754	/(H12H9A) 38.336	0.43 H
4	8.584	/(HnH9A) 38.400	
6	7.852	/(H12H9A) 38.912	0.14 H
5	7.974	/(HbH9A) 39.296	0.736 H
5	7.998	/(HbH9A) 40.064	
5	7.819	/(H12H9A) 40.832	1.356H
6	7.697	/(H12H9A) 41.984	0.14 H
4	8.594	/(H„H9B) 42.432	
In the 1H-NMR (100 MHz) spectra of 1 the NH group signals in the 8 8.637-8.514 and 8 8.077-7.646 range confirm the 1A, 1A', 1Ar 1A2 and 1A'r 1A'2 1A'3
resonance structures, respectively (Table 11).1 The signals at 5 8.594 /(H11H9B) 42.432 Hz, 5 8.584 /(H11H9A) 38.400 Hz, 5 8.528 /(HnH9A) 37.280 Hz and 5 7.998 /(H13H9A) 40.064 Hz (spectra 4, 5 Table 12)1 point to the transition of A' < A and A1 tautomers as well as to the rapid exchange at the NH group hydrogen of structures A A'.
4.	Conclusions
The 1H NMR studies (100MHz) of (3-phenyl-allyl-) (5-pyridin 2-yl-[1,3,4] thiadiazol-2-yl)-amine support the A(I) < A(I)' < A(I)'a, A(II) < A(II)' < A(II)'a structures. The intensities of the signals of N-H proton in the range from 8 7.890 to 7.190 confirm the balance of two tautomeric forms A(I)' ^ B' or A(II)' ^ C(II)' in the solution. Double signals of the NH proton in the 1H-NMR (100 MHz) spectra of 2 (Figs 7-10, Table 10) confirm both tautomeric forms. Because of the rapid exchange of NH group hydrogen in this case the pathway of the proton transfer via solvent may take place.
5.	References
1.	L. Strzemecka, Int. J. Mol. Sci., 2006, 7, 231-254.
2.	L. Strzemecka, D. Maciejewska, Z. Urbanczyk-Lipkowska, J. Mol. Struct, 2003, 648, 107-113.
3.	P. Fremont, H. Riverin, J. Frenette, P. A. Rogers, C. Cote, Am. J. Physiol., 1991, 260, 615-21.
4.	A. D. Kenny, Pharmacology, 1985, 31, 97-107.
5.	A. C. Potts, U. K. Britt, Pat. Appl., G B 2, 223, 166 (Cl A 61 k 31/425) 04 Apr 1990.
6.	K. Miyamoto, R. Koshiura, M. Mori, H. Yokoi, Ch. Mori, T. Hasegawa, K. Takatori, Chem. Pharm. Bull., 1985, 33, 5126-9.
7.	S. M.Cohen, E. Ertruk, A. M. Von Esch, A. J Crovetti, T. G. Bryan, J.Natl. Cancer Inst., 1975, 54 (4), 841-50.
8.	M. Miyahara, M. Nakadate, S. Sueyohi, M. Tanno, M. Miya-hara, S. Kamiya, Chem. Pharm. Bull., 1982, 30, 4402-6.
9.	M. G. Mamolo, V. Falagiani, D. Zampieri, L. Vio, E. Banfi, Farmaco, 2001, 56, 587-92.
10.	A. K. Gadad, S. S. Karki, V. G. Rajukar, B. A. Bhongade, Ar-zneim. Forsch., 1999, 49, 858-63.
11.	F. Cleirci, D. Pocar, M. Guido, A. Loche, V. Perlini, M. Bru-fani, J. Med. Chem., 2001,44, 931-6.
12.	M. Barboiu, C. T. Supuran, L. Menabuoni, A. Scozzafawa, F. Mincione, F.Briganti, G. Mincione, J. Enzym. Inhib. Med. Chem., 2000, 15, 23-46.
13.	G. Mazzone, R Pignatello, S. Mazzone, A. Panico, G. Pen-nisi, R. Castana, P. Mazzone, Farmaco., 1993, 48, 1207-24.
14.	J. M. Cox, T.R. Hawkes, P. E. Bellini, M. Russell, R. Barrett, Pestic. Sci., 1997, 50, 297-311.
15.	F. Zucchi, G. Trabanelli, N. A. Gonzales, ACH-Mod. Chem., 1995, 132, 579-88.
16.	L. Strzemecka, Pol. J. Chem., 1990, 64, 157-166.
17.	C. Lee, W. Yang, R. G. Parr, Phys. Rev., 1988, B 37, 785-9.
18.	A. D. Becke, J. Chem. Phys., 1993, 98, 5648-52.
19.	M. J. Frisch, G. W.Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Jr. Montgomery, R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A. Peters-son, P. Y. Ayala, Q Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, A. G. Baboul, B. B. Stefanov, G. Liu, A. Liashenko,
P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P. M. W. Gill, B Johnson, W. Chen, M. W. Wong, J. L. Andres, C. Gonzalez, M. Head-Gordon, E. S. Replogle and J. A. Pople, Gaussian 98, Revision A. 7, Gaussian, Inc., Pittsburgh PA, 1998.
20.	L. Strzemecka, Annales UMCS Sectio AA, 1999/2000, vol. LIV/LV, 379-392.
21.	L. Strzemecka, Annales UMCS, Sectio AA, 1999/2000, vol. LIV/LV, 363-377.
22.	A. Krzan, J. Mavri, Chem. Phys., 2002, 277, 71-76
23.	M. H. M. Olsson, J. Mavri, A. Warshel, Phil. Trans. Roy. Soc., 2006, B, 361, 1417-1432
Povzetek
Določili smo strukturne oblike iona ali radikala spojine (3 - fenil - alil-) (5 - piridin - 2 - il - [1,3,4] tiadiazol - 2 -il)-amina 2A(I) 2A(I)' 2A(I)'a, 2A(II) 2A(II)' 2A(II)'a s pomočjo njenih 1H (100 MHz, 500 MHz), 13C in 15N NMR spektrov in B3LYP/6-31G** izračunov. Tavtomerno ravnotežje 2A(I)' ^ 2B', 2A(II)' ^ 2C(II)' je določeno s pomočjo 1H NMR spektra (100 MHz).