Symmetric Stretching Mode:

The molecule is alternately stretched and compressed, both C-O bonds changing simultaneously as in figure.

Figure: The symmetric stretching vibration of the carbon dioxide molecule

Plainly the dipole moment remains zero throughout the whole of this motion, and this particular vibration is 'infrared inactive'. The frequency of this mode is

$$\nu_1 = 3.99 \times 10^{13} s^{-1}$$

$$= 1330 cm^{-1}$$

The associated quantum number is denoted by $v_1$.