Introduction
Basicity of a base B in solvent S is defined using eq 1 and is expressed as dissociation constant K a
of the conjugate acid HB + of the base B or more commonly its negative logarithm p K a
.
HB
+
+ S  B + HS
+
K a
 a ( HS

)
 a ( B ) a ( HB

)
(1)
(2)
To exclude the necessity for measuring the solvated hydrogen ion (HS + ) activity (its measurement is problematic in nonaqueous solvents) we studied the equilibrium between two bases B
1
and B
2
:
B
2
+ HB
1
+  HB
2
+
+ B
1
(3)
This equilibrium refers to the relative basicity of the two bases B
1
and B
2
, which is denoted as  p K a
and is defied as follows:
 p K a
 p K a
( HB

2
)
 p K a
( HB
1

)
 log a ( HB
2
 a ( HB
1

)
)
 a ( B
1
 a ( B
2
)
)
(4)
As can be seen the activity of HS + is excluded from the equation. If it is assumed that the ratio f (HB + )/ f (B) is the same for both bases (see ref 3). Then the eq 4 transforms into the following form:
 p K a
 p K a
( HB

2
)
 p K a
( HB
1

)
 log
[HB
[HB
1
2


]
]

[B

[B
2
1
]
]
(5)
The equilibrium concentrations of the species are square bracketed.
The measurements were carried out using our previously developed method [1, 2, 3
@õigeks]. The method consists of UV-Vis spectrophotometric titration of a solution, where both of the bases are present, with an optically transparent acid or base.

Chemicals
The synthesis and purification of reference compounds P1 , P2 , P3 and P5 are described in reference [1, 4, 5, 6, @viited õigeks], compounds P7 in reference [@] and compounds P4 and P6 are used first time in this work.
4-CF
3
-C
6
H
4
-P
2
(pyrr) ( P7 ) – df
4-Br-C
6
H
4
-P
2
(pyrr) ( P4 ) – df
@Siia peaks tulema kahe uue fosfaseeni sünteesi kirjeldus jnejnejne
Acetonitrile (AN) (Romil, > 99.9%, Super purity Solvent (Far UV), water content < 0.005%) was the same used in previous works [1, 4] and was used without further purification. The water content was determined by coulometric Karl Fischer titration to be about 0.004%.
Solutions methanesulfonic acid (MeSO
3
H) (> 99%) was used as acidic titrants. Solution of phosphazene base EtP
2
(dma) (> 98 %) [] was used as basic titrant.
Measurements
The spectrophotometric titration method used in this work is the same as described earlier [1,
2, 5@] i.e. the simultaneous titration of two free bases of comparable basicity with an acid was carried out and the UV-Vis spectrum was recorded after each addition of acidic titrant.
Also, both bases were titrated separately. Professional glovebox (MBraun) was used to ensure that the environment is free from humidity and oxygen. Perkin Elmer Lambda 40 spectrophotometer was equipped with an external sample compartment, situated in the glovebox. The cell compartment was connected to the UV-Vis spectrophotometer by means of two quartz fiber optic cables. Glassware used during the experiments was heated at 150 ºC for at least six hours and then cooled in glovebox. Concentrations of measured bases were in
5  10 -5 M range during the titration experiments, concentration of acidic and basic titrants were usually around 5 · 10 -4 M.
Calculation Methods
The  p K a
values for the pairs of bases were obtained from the UV-Vis spectra. From each titration experiment of the mixture of bases, the  p K a
was determined as the mean of 10-20 values.

The substituted guanidine bases measured in this work have suitable for our measurements spectral properties in the wavelength range of 215 to 235 nm. All the phosphazene bases used as reference bases had an isosbestic point (the absorbance of protonated and deprotonated forms is the same) in the range of 220 – 230 nm in AN solution. Therefore the wavelength of the reference base isosbestic point can be used for measuring the protonation level of the guanidine bases in the solutions.
The
 p K a
values were calculated as follows. When two partially protonated bases are in the same solution, the Lambert–Beer law for the net absorbance at wavelength λ can be written
(the optical path length, 1 cm, is equal for all substances and is included in A
λ
, absorbance caused by solvent is compensated and concentrations normalized to unity for the bases are used [HB
1

]

1

[B
1
] ja [HB
2

]

1

[B
2
] ):
A
λ 
[HB
1

]
ε λ
HB

1

[B
1
]
ε λ
B
1

[HB
2

]
ε λ
HB

2

[B
2
]
ε λ
B
2
(6)
The
 x

values are the absorptivities (referring to the normalized concentrations) of the respective species x at wavelength

.
The wavelength (denoted as



at which the reference phosphazene bases (denoted as B
2
) have the isosbestic point is selected as the first analytical wavelength. The absorbance of protonated and deprotonated forms of the guanidine bases under investigation (denoted as
B
1
) are different at that wavelength. Thus the absorbance of the solution in wavelengt

is:
A
λ 
[HB
1

]
ε λ
HB

1

[B
1
]
ε λ
B
1
 ε λ
B
2
(7) and the protonation level of guanidine is:
PA
B
1

[
[ HB
1
]
HB

1
]

[ B
1
]

A
A


B
1
B
1


A

A

HB

1
The protonation levels of the phosphazene bases are found at longer wavelength

2
(in the
(8) range of 270-290 nm) where the guanidine bases do not absorb. The absorbances of the protonated and deprotonated forms of the phosphazene bases are strongly different at that wavelength. The absorbance at

2
is expressed as:
A
λ 
[HB
2

]
ε λ
HB

2

[B
2
]
ε λ
B
2
(9)
The protonisation level for B
2
can be written:

PA
B
2

[
[
HB
2

B
]
2

]
[ B
2
]

A
A


B
2
B
2


A

A

HB

2
The calculation of
 p K a
is then as follows:
 p K a
 log
PA
B
2
PA
B
1

( 1

PA
B
1
)

( 1

PA
B
2
)
(10)
(11)
Also, some other calculation methods (described previouly in refs @) were used and good agreement between different approaches was found: the differences were in general not higher than 0.05 p K a
units.
The absolute p K a
values were calculated as in the previous papers [1, 2] by minimizing the sum of squares of differences between directly measured
 p K a
values and assigned p K a values while keeping the p K a
values of other bases constant. In this work, two minimization steps were carried out. First, p K a
values of two new phosphazene bases (P@ and P@) as good references bases were calculated while keeping the p K a
values of other bases constant and then p K a
values of guanidines were calculated while keeping the p K a
values of all other bases constant, also those two new phosphazene bases. However, it should be stressed, that the absolute p K a
values of bases given in Table 1 are not as accurate as the relative p K a values. The consistency of the results can be assessed using a standard deviation s as defined by eq @6, s
 n m u
 n c
(12) where n m
= 205 is the number of measurements, n c
= 101 is the number of p K a
-s determined.
For our results, the global standard deviation s = 0.03 p K a
units. The uncertainty estimation of p K a
measurements according to this approach has been carried out recently [@ACA viide]. Based on the results of that study we estimate the standard uncertainties of the obtained p K a
values if interpreted "in the framework" of the AN basicity scale (that is, uncertainties to be used when comparing the different p K a
values from the scale to each other) as 0.06 p K a
units. The standard uncertainties of the absolute p K a
values "detached from the scale" (that is treating them as negative logarithms of equilibrium constants) can be estimated as 0.2 p K a
units.

@ skaala.
@Pilt pyrride kohta
References@@@viited korda teha
1.
Kaljurand, I.; Rodima, T.; Leito, I.; Koppel, I.; Schwesinger, R. J. Org. Chem. 2000 ,
65 , 6202-6208.
2.
Leito, I.; Kaljurand, I.; Koppel, I. A.; Yagupolskii, L. M.; Vlasov, V. M. J. Org.
Chem.
, 1998 , 63 , 7868-7874.
3.
Leito, I.; Rodima, T.; Koppel, I. A.; Schwesinger, R.; Vlasov, V. M. J. Org. Chem.
1997 , 62 , 8479-8483.
4.
Inamo, M.; Kohagura, T.; Kaljurand, I.; Leito, I. Inorg. Chim. Acta , 2002 , 340 , 87-96.
5.
Rodima, T.; Kaljurand, I.; Pihl, A.; Mäemets, V.; Leito, I.; Koppel, I.
J. Org. Chem .
2002 , 67 , 1873-1881.
6.
Rodima, T; Mäemets, V; Koppel, I. A.
J. Chem. Soc., Perkin Trans. 1 2000 , 2637-
2644.
7.
Kaljurand, I.; Kütt, A.; Sooväli, L.; Rodima, T.; Mäemets, V.; Leito, I.; Koppel, I. A.
Extension of the Self-Consistent Spectrophotometric Basicity Scale in Acetonitrile to a Full Span of 28 p K a
Units: Unification of Different Basicity Scales. J. Org. Chem.
2005 , 70 , 1019-1028.