Maria Bárbara was about four years from marriage in 1729 when this portrait was painted. Her Wikipedia article includes this, "In 1729 at age 18, she married the future Ferdinand VI of Spain, two years her junior. Her brother Joseph married Ferdinand's half-sister, the Spanish Infanta Mariana Victoria, previously rejected bride of Louis XV. The double marriage was meant to repair the Portuguese-Spanish alliance. Scarlatti followed her to Madrid on her marriage and remained with her, composing hundreds of harpsichord sonatas for her. During her husband's reign, Barbara presided over magnificent parties and concerts at the Royal Palace of Aranjuez, which was her favourite retreat.
Although Barbara was not beautiful — her homely looks were thought by observers to cause the prince a visible shock when he was first presented to her — 'Ferdinand the Learned' (as he was called) became deeply attached to her, sharing her passion for music. Ferdinand soon became dependent on Barbara for advice and support; when he ascended the Spanish crown as Ferdinand VI, the French ambassador noted 'it is rather Barbara who succeeds Elizabeth (step-mother of Ferdinand) than Ferdinand succeeding Philippe.' Barbara was well-educated, adept at languages, an avid reader of many varied subjects, and had a love of music.
The couple produced no children, though Barbara had a stillborn son in 1733. Barbara suffered from severe asthma for most of her life. She became quite overweight later in life. Barbara's death was said to have broken her husband's heart."
She was Portugal's contribution to a reciprocal royal marriage. The Princess from Spain was Mariana Vitória.